A few of the guys have already mentioned it, but it bears repeating:
It's on record from both Diaz boys that they don't necessarily enjoy fighting. They love bjj, they love the training, the community, their gym and their crew but if they didn't have to fight for the money, they wouldn't.
In Nate's case, he probably literally got paid 20x to fight McGregor what he did to kick the everloving shit out of Michael Johnson for 15 minutes. Neither Diaz is educated worth a fuck, but they aren't stupid. One more red panty night with McGregor and Nate never needs to work again. He might not necessarily need to right now.
But I thoroughly expect that if the rematch happens, win OR lose, Nate would take his gigantic Sweepstakes cardboard check, cash it and ride off into a hazy sunset.