This is such an idiotic debate. ALL game series add new worlds, characters, creatures and items as they go along. That's the entire point. That's what a series is for. The only difference is that in Pokemon, all of the oldest creatures can be brought into the newest games. This is slightly unusual for a video game, but the accepted standard for things like trading card and miniatures games. Magic: the gathering has THOUSANDS of monsters now, and is still going strong with each new set of cards eagerly anticipated by its fandom.
There are limitless possible concepts Pokemon, can be based on and limitless new strategies and levels of gameplay that can be added. 4th gen did not "tie up" ANYTHING. There is nothing to tie up. There is always room for expansion and expansion is always welcomed by rational fans of any franchise. How can there be "too many" Pokemon? You aren't required to collect them all. Your goal is to assemble a team that you enjoy using, and the more there are, the more options you have.
Furthermore, I can think of hundreds of extremely obvious concepts they haven't exploited for Pokemon at all. Animals as commonplace and popular as ants and dolphins don't even have Pokemon yet.