It also isn't like this is happening in Game Freak right now
"Yeah, um, we are creating a sequel to Black and White"
"Are you going to add new pokemon?"
"Um, sorry, we can't, we designed it so that our game engine couldn't be flexible enough to add new pokemon, compared to other game engines from other game franchises that adds new characters, weapons, and maps"
But then again, the game engine is indeed not flexible to add new pokemon, much so that they need to create a brand new one to add new pokemon.
Those who admit that it is impossible, that is, based off limitations, that there will be new pokemon, is admitting that Game Freak doesn't make flexible game engines, the DS isn't with the future, ahem DLCs, despite it selling a lot, and also thinking way into the past, much so that the norm for a "patch" is to release a new game, and also hasn't seen how competing companies such as Sony and Microsoft are distributing DLC and patches via their stores, and even Nintendo distributes content through whatever store they have, so it wouldn't be impossible if Gamefreak distributes new Pokemon as DLC. It is just that richer kids have more pokemon than poor kids. It isn't like there aren't 2 pokemon that are functionally the same.
I'm only targetting at people who say that Game Freak can't add new pokemon, not those who say that Game Freak won't add new pokemon.