Lose money? Hah, you are deluded. GTA V has most likely more owners on Steam than only on RGSC, and for those copies they get full profit, unlike all CD-keys that most likely are from cheaper retail markets. I certainly don't think they would lose money on it. GTA is much bigger than Destiny and they didn't go exclusive.
Rockstar doesn't have a service already out there being used by hundreds of millions of people to put their game on.
Do you honestly believe that if GTA5 was proprietary rockstar launched on PC that it WOULDN'T have sold a zillion copies?
Because that's what you're asserting, with literally no way to back that up