I seriously doubt it'll be uncrackable. It might take awhile, but if the game is good enough to be deemed 'worth it' by the scene, it will get cracked eventually. Even forced online like Diablo 3 and Simcity, private servers could have been made, but the games were so terrible that nobody gave enough of a fuck to do it. People will give enough of a fuck with GTAV.
There's thousands upon thousands of PC games that have been released over the years that someone could go back and play whilst they wait for a crack rather than eat up GTAV on release day and I think many Pirates will do just that. These people simply aren't willing to spend money on games, I'd say a strong portion won't cave for the latest GTA.
Ultimately we're all throwing around a lot of assumptions, just have to wait and see what happens, but I just think chances are this DRM won't make the majority of Pirates go buy the game. Or as you said, they'll wait until a heavy, heavy sale, which I guess is better than nothing but still not what Rockstar/2K is looking for, they want to maximize sales early.