Pokemon's certainly popular for a game, but I wouldn't even call it mainstream. (The fact that a non-mainstream game can sell 15 million copies should be an invitation for companies to make mainstream games. Alas.)
It's a figure of speech. Certainly, everything has some niche.
The Gamecube is a perfect example of trying to appeal to new generations with no regard for what came before. Pokemon will eventually drop out of the mainstream (or whatever subset of mainstream it's currently within).
If certain Pokemon aren't eliminated eventually, the bad ones will clutter and flood the games eventually. If they are eliminated, you alienate fans of those Pokemon. Naturally, Pokemon must enter decline (like all things in life).
It's begun to reach it's threshold. Ice Cream Cones and Mystical Sage Villians show that what Pokemon is has been diluted. People no longer talk of their favorite Pokemon but their least favorite. It can never recapture its old magic. It'll hobble on for a few generations and combust.
Sure, you can say that if it makes you happy doesn't annoy me.
Pokemon can't continue forever in it's present form. The series' actions have established a few canon rules: No content (Pokemon, items, moves, abilities, etc.) may be removed. Additional content must be added each generation, to take precedence over old content.
These appeared to be logical conditions. (Would anybody expect new games with nothing 'new' to be successful?) These assumptions, however, mean that Pokemon will eventually reach 100,000 Pokemon with 35 generations. That's untenable.