Well, I agree with some of that- Anderson has aged, and he probably has lost a few milliseconds of speed, but don't forget that he was looking just as potent as ever only 2-3 years ago. Sometimes guys just age quick, and maybe Weidman really stole his soul out there, so maybe that's what's happening here, but until I see him fight someone else I'm gonna assume Anderson is still Anderson. He's training partners are all commenting on his dedication, motivation, and the fact that he's still kicking "fucking hard". They're obviously biased but it seemed genuine.
I'm a fan of both guys, and I would probably jump out of my seat yelling at the TV and laughing hysterically if Diaz beat Anderson, but I just don't think it's a good matchup for Nick.
Also, you keep referencing super old fights. Anderson has continued to evolve throughout his career and he really wasn't the same fighter he is now (or at least, prior to Weidman). Don't forget, it's also been years since Diaz beat ANYONE, let alone even a top WELTERWEIGHT. Paul Daley in 2011 almost KO'd him if it weren't for dat chin. His next fight was against BJ Penn who really doesn't belong at 170lbs in the first place. That was 2012, the same year Anderson defeated a juiced up Bonnar and Chael with uber knees.