4. They just gave up after Johto or something.
*raises hand*
This is me. I loved the first and second generation of games, but Ruby and Sapphire killed it for me. I bought Diamond Version, but never play it. I gave up on the anime around the Orange Island period. I haven't played the more recent games lately to give any kind of thorough critique on them, but I felt that they weren't innovating enough and the quality of nearly everything (designs, worlds, attacks, characters, types, art) was going down.
As for being too childish, Pokemon is a surprisingly complex thing. It's complex in that it is as complex as the player wants it to be, if that makes any sense. You can go through the games playing like a numb skull, with your level 89 Charizard and your level 6 everything else, or you can pour more effort and brains into getting a well balanced and solid team. For those that want to do serious strategizing, the option is definitely there, despite the childish guise.
Though I'm not nearly as big a fan as I used to be, I don't have any real beef with the series, so I do want to play my Diamond Version. I just need to get around to it.