We already went over this, though. Diddy loses to Snake and Wario, and goes even with a crapton of other characters, such as Luigi, Peach, TLink, and Falco. There are a lot more characters Diddy goes even with, as well, but I can't remember them right off the top of my head.
And don't forget, Diddy actually has bad stages that he can be counterpicked to. There are AT LEAST two such stages against Diddy's opponent's one stage ban; Brinstar and RC. There may be some other bad Diddy stages, but those two come to mind right off the bat. He's not like Meta Knight, where stage selection doesn't even matter because MK will **** you on any stage you CP him to regardless...
It's not like Diddy is the "cover all options" character Meta Knight is. He loses to some characters and doesn't even solidly beat a large portion of the cast, plus he can be counterpicked to a medley of different stages.
In addition, if, by some random quirk of fate, Diddy actually DOES somehow become the next Meta Knight, people will go ahead and learn proper item control, as well as the Single Naner Lock for their character(s), thereby shifting virtually every matchup slightly towards the non-Diddy characters' favors. That's how a meta-game works. That's how it evolves. You are not going to see this kind of progress with Meta Knight still legal.