1. I don't exactly know what this supposed "new breed" of fighter is. That term has been thrown around for at least 8 or 9 years now and Diaz has still been competitive (when he's not suspended for weed) during that stretch.
2. We've already seen Nick lose, a lot. I don't get the comparison between him and Ronda in that sense. Ronda was built up as an unstoppable machine, but her aura of invincibility is now gone. Diaz has been a perennial contender but has never reached the level that Ronda did (that's not to say Ronda is more talented than Nick, because she's not, just that he's never had a lengthy title run in the UFC like Ronda, nor has he enjoyed the same level of mainstream popularity).
3. Diaz is 33 and likely past his physical prime, he's also been extremely inactive for the last 5 years. If he's "lost it" it's probably because of physical limitations and the fact that he has put a lot of miles on his body, and not because of some mythical "new breed" of fighter that he'll be up against.