It makes good sense to keep the game as present as possible, for as long as possible.
Regardless, GTA 5 is the best tool Rockstar has in its arsenal to hype GTA 6, even if GTA 6 might not actually need all that much hyping, at the end of the day. It might be years from now, or it might not be. And we remember, too, that GTA 6 is out there, at some stage in development, and that the day will eventually come when it actually comes out.
