This is a highly unrealistic expectation of Rockstar. Loading the entirety of the USA would have its issues. Even just driving from one city to another could be a real chore, especially if it's just filled with country side and nothing much to do in between such as in states like Utah and Montana. The Crew had the same issue, they didn't include small towns that much and only put in major cities such as LA, Boston, Huston, Chicago, etc. Do you expect Rockstar to visit each and every city/town in the US to make a detailed version of it? That'd be great, but it's completely absurd. Even Ubisoft downscales the size of the US majorly compared to real life. It takes about 60 minutes to drive across the entire country in the game but in real life it'd take several days (about two and change of nonstop driving).
Second, the hardware requirements would be out of this world, even with the best coding that Rockstar could manage. By then, we should hopefully see consoles either gone, or rolled into services that stream the content to you from a supercomputer, similar to how NVIDIA's GRID technology works. But just imagine, a super-sized Grand Theft Auto VI playable on the PC, or on a vr headset. The main problem I see with making a whole USA (to scale & accurate) is how resource intensive it would have to be. Unless everytime you drive out of an area you want to wait in a loading screen this is unavoidable.
Another thing I see Rockstar doing is Vice City mainly because they'll probably title "Grand Theft Auto: VIce City" or some cringy thing like that.
Edit:
Didn't see that someone mentioned this already