I disagree. Yes, you need to have enough Pokemon so that people have options, but you don't need Pokedexes anywhere near the size of XY to do that. ~200 is more than enough.
Kalos has 457 Pokemon available in its Pokedex. That is ridiculous; we didn't even have that many Pokemon until Diamond and Pearl!
In XY (and BW2 for that matter), I just felt like I was drowning in Pokemon. Every area had so many Pokemon available, and tons of Pokemon were unique to a particular route. It just comes off like the world isn't big enough for the Pokedex they're using.
But, besides all that - it doesn't even matter. Pokemon games dropped trade restrictions a long time ago. If they go for a smaller Pokedex, and don't happen to include a particular Pokemon you want to use, that doesn't stop you from using it. You just have to trade it in.