Problem for Diaz striking is his game is predicated on his opponent not being able to disengage without taking some shots. If you look at the elite strikers he took down they were guys who liked to get inside and throw hooks and the like. Daley, Cyborg, Zaromskis. And every time one of those guys is getting in to or out of striking range they're going through a barrage of fists to get there.
So, when was the last time you saw GSP striking inside? Trading hooks and the like. Most of Nick's striking is nullified before they even step in the cage.
So where does that leave us? His BJJ. And here's the dirty little secret: Nick Diaz has never submitted a guy with world class BJJ defense, or even particularly high end BJJ defense. GSP has this. He hasn't even really been threatened with a close submission since the Hughes armbar a hundred years ago. It's not that Nick's sub game is bad, that would be an idiotic statement, but he's never proven he's a guy that will create openings for submissions on his own against a truly elite opponent.
All of this is ignoring that to work his BJJ game at all he'll be reliant on GSP taking it to the ground, because Nick doesn't even seem to believe in takedowns. How likely is it he'll suddenly be able to take down maybe the best wrestler in MMA?
None of this means Nick can't win, that he won't land that one stiff punch he needs to wobble GSP and pounce. It just means that on paper is is not a particularly scary match-up for GSP. If he fights his fight he's got every advantage, and there's no good reason to think he won't be able to fight his fight.