I don't see it happening either. The majority of people who will have a next-gen console within the next two years will probably have played GTA V already. It will take years for PS4 and Xbox One to reach the install base that current-gen consoles have. Why waste the money?
Agreed. Unless both Xbox One & PS4 starts building up huge install bases from the start of their launches, there's a very slim chance that a GTA V port will happen.
It's best to start with GTA VI instead with 2-3 years into their lifespans.