We will be going the opposite route.
First off, we need to clarify certain benefits of additional Pokémon, then rebuke common claims against them, and then rebuke uncommon claims against them. There won't be any "debunking" here, however, since this is a topic open to debate. I merely chose to defend the opposite route to play devil's advocate, and because I genuinely believe it.
This will be more of a rant, however, so many things will sound either obscure or swallowed within words.
What are the benefits of a large introduced dex?
1.Introduction of new members of each type:
Currently, with the Fairy-type, there are 18 types, with each generation showcasing at least one representation of each type. Sometimes, this is limited to only one species(Dark-type representative of Generation I being Mega Gyarados, Dragon and Ghost representatives of Gen II being Kingdra and Misdreavus respectively), but more often than not. Most famously however, Gen V introduced at least three members of each type to fit the three Pokémon selection of its Gym Leaders. If such a principle applied to every generation we would need:
18*3= 30+24=54 new Pokémon.
This increases when we consider that on average most Pokémon have two stages, making this 108 immediately. If we use traditional 4 Pokémon instead of current 3-2 Pokémon however, things go out of whack;
18*4*2=18*8=80+64=144 new Pokémon.
That's a lot of species. While this seems absurd, it is actually necessary, as many of the types, such as Ice, Ghost, Dragon, have pathetically low members compared to Normal/Water, and there are several mid-level types such as Electric, Ground, Fire etc. who would also enjoy a greater distribution.
No matter how you look, however, to increase the variety for each type, a large, fat dex is necessary. We are not even talking about theoretical monotype teams that are desired by some parts of fandom, since achieving that for all types in any given generation is a functional impossibility(it would need at least 18*6 different species at least, 18*12 according to general distribution of species, and 18*18 to provide anyone with a strong three stage team of each type).
2.Introduction of new gimmicks and signature moves:
This is more of an analogical question than a factual one. In Generation VI, we had possibly the highest amount of signature moves, so a large dex is not a necessity for large moves. However, it allows species that are dedicated to those gimmicks to flourish, or more commonly, miserably fail, so the rest of the Pokémon can simply be standard Pokémon, and not be species that are forced with new radical abilities and moves.
This would be both positive and negative. For example, without the nonsense of Symbiosis/Flower Veil Florges could have become a powerful defensive tank, but with its stats, it is one of the species to make its abilities work. Generally however, since such variations exist only for sake of existing, it is probably better to continue to sacrifice few species to these failed experiments than to try to make them work.
And if they work, then they can be conceivably expanded to more species. A larger dex would allow old and new signature moves and abilities to become new again, and with better stats to work with. Alternatively, their distribution to other species might force them to update themselves, such as how Knock Off went from a useful utility move to a primary offensive move.
3. Marketing, marketing, marketing!
Not only a larger dex means more options, it also means more toys. Obviously, only a specific set of species will be decided to be the primary marketing ploy, specifically those that belong to the animé main cast, but others will still remain marketable.A large selection of Pokémon also increases the chances of them being selected by certain audiences, thus increasing the chances of an unexpected popularity.
Large dex also means increased amount of filler for the animé series and serial manga, if they don't old species for filler(see X/Y and D/P, both of which old species for filler).
4.The element of surprise:
This is a subjective statement, however, a common one. One of the big desires of most(but not all) Pokémon fans is to see a new species at least in every new area. Let's count, for example, the list of Wild Pokémon areas for X/Y:
- Route 2
- Route 3
- Route 4
- Route 5
- Route 6
- Route 7
- Route 8
- Route 9
- Route 10
- Route 11
- Route 12
- Route 13
- Route 14
- Route 15
- Route 16
- Route 17
- Route 18
- Route 19
- Route 20
- Route 21
- Route 22
- Ambrette Town
- Victory Road
- Terminus Cave
- Shalour City
- Santalune Forest
- Reflection Cave
- Pokémon Village
- Lost Hotel
- Frost Cavern
- Glittering Cave
Now, less go through some common objections;
1.A shorter dex can provide better quality of Pokémon:
This is seemingly true for X/Y and D/P, and only partially true for G/S/C. This idea purports that because of the lesser dex, GameFreak can focus on the few species there is and give them significant moves, abilities and stats. And yes, this is very much true. However, it is ignoring two aspects:
a.Initial success is important for further success, but the success of a species increases geometrically with alternate forms, stat updates and move updates:
Pokémon series is a not a creationist series, which each generation being created independently from the previous one. It is a constantly evolving, dynamic series that takes a lot of inspiration and flaws from its past. It is less "eternal recurrence" and more "endless forms most beautiful".
Such gradual evolution can be best seen on species such as Mr. Mime, which gained a higher Sp. Defence from its original stats in Gen II, completely new abilities in Gen III, physical/special split which it partially lost and partially benefitted in Gen IV, and a whole new type in Gen VI.
With this in mind, we can't say anything about a current performance of a species besides when we are talking about its performance in its specific generation. This is relevant, very much so, as good species tend to be become even better in time span(Tyranitar and Lucario both have "fast-track evolutionary speeds" so to speak), but the ones that aren't also increase in complexity and quality over time, even if they might lose a lot of their comparative advantage through time cycles(Tauros in Gen I and Politoed in Gen V are notable examples of kings who are lost to time).
b.Larger generations means more fully evolved species:
Another big issue is the lack of three stage Pokémon. In large dexes, this is not an issue, as not dependent on previous dexes, these generations must create new, iconic species from whole cloth, not give evolutions to powerful species. This is generally a good thing, even if cross-generational evolutions tend to be better statistically, as these new three stage species are more varied, more common and more easily accessible than their cross-generational counterparts. And unlike their cross-generational counterparts, they actually have a duty to represent their respective generation and have to be better than most of their generational brethren, while cross-generational Pokémon only need to update their previous stage(or, in a lot of cases, exist alongside what was there before, which cuts down their quality significantly. Megas seemingly fixed this issue, but only for now).
c. Shorter dex cuts down on filler:
One of the major complaints within the fandom is how many dexes are complete with filler Pokémon, that is, species that only exist to fill in certain route spots and not much else.
However, as we discussed before, this is in inevitability. Most specifically, however, Pokémon is not a competitive game predominantly(or at least, wasn't, until Generation III/IV). This means that many common species, such as Watchog, Vivillon, Masquerain et. al are necessary. While one can try to make them good, as D/P and X/Y did, one can also make them, and forget them forever to focus on real species, such as Flygon, Milotic, Haxorus, Hydreigon, Mienshao. The latter option might actually be beneficial, because the former species have to work within two different spheres of success, being a crap Pokémon to be beaten, and being a powerful species to beat, whereas others are difficult to tame both in-game and in competitive scene.
My personal choice would be a combination of both, with earlier species having either obvious but uncommon weaknesses(Trash Wormadam), or having alternate forms and Hidden Abilities that make them as powerful as pseudo-legendaries(Diggersby), or being very powerful but one dimensional(Staraptor). But this is difficult to balance within competitive metagame, and most of these early species will also be made common animals in most alternate canon and in-game scenes.
2.A smaller dex will be better received:
This seems to be controversial issue. While BW and RB are well received for their original Pokémon, so has DP. However, BW and RB's reception seems to be general, whereas DP's reception might be largely influenced by the periodical hype towards the two generations back.
However, a lot of fan games have historically justified their existence on a large selection of new Pokémon, and many people were hyped up with the prospect of a large dex for XY(which was never promised, or happened) so it seems to be a popular idea.
Now, let's go with the uncommon claims against this idea:
1. The list of Pokémon is getting larger and larger, it is harder to keep with every new Pokémon:
This is generally a rebuke against new Pokémon in general, but it also used against large introductions. This to me, is a bit ridiculous argument, because as we discussed before, introduction of new species also forces new gimmicks, which can then be used by older species.
However, increasing complexity is a sound argument, especially since Sugimori himself mentioned it. Now, as far as I'm concerned that's just nonsense, certain ideas will always fail and others will always succeed, so there really isn't a perfect metagame(in-game or competitive), and our increasing complexity, as I have mentioned before, has been positive. But for its "confusing" nature, well, certain failed experiments can simply be removed. But not the species, and certainly not the method.
If there are any arguments you have, please feel free to tell them. I'm out of words.