Truth is, initially both were to be in Brawl. Mewtwo was never REPLACED. A bunch of people hate Lucario for that, when the poor little guy didn't do anything!
[link]The Subspace Emissary took more time than planned, and so Mewtwo was dropped. I like them both a great deal, and it is a fact that dropping characters from a fighting series is stupid. (except that darned jigglypoop.)
If in the next Smash Brothers, Lucario is dropped, I won't buy the game. That's all there is too it. He's my only new main, and probably my best. I'd hate to see him go.