I actually disagree with this. I'd rather they have less Pokemon in future games rather than more.
It really annoys me how many Pokemon from previous gens they put in the new ones. The different regions in Pokemon don't feel that different if they all share the same Pokemon. I really loved gen 5 and Isshu for that reason. It only had new Pokemon, none of the old ones at all, which really made it feel new, unique and interesting. Unfortunately that doesn't seem like something that'll ever happen again, especially not with the recent gen 1 nostalgia drive Gamefreak have been on. But I'd still love more Pokemon that can only be found in a specific region, nowhere else, that makes those Pokemon, the region they're in and the game more interesting IMO.
Why would you want less content in the games you play?
You know it deserves the GOTY.