GTA generally is pretty good about keeping things close to you around, e.g. looking backwards won't usually remove traffic in front of you. The most obvious case are NPCs that aren't "relevant" anymore, e.g. people that have places around the city that depend on time of day. If their "active" time ends, they'll start wandering off from their location. If you follow one, you can for a long time, but if you then even look away for a split second they'll be removed, presumably because the game tracks them as something it wants to unload after their useful time has ended, but won't do it while you look.
In earlier GTAs like San Andreas there is an explicit difference between "traffic in the distance" and "traffic close by" - the former will be removed if out of sight, doesn't have full-fidelity 3D models, apparently doesn't have physics applied to them, ... Once they get close enough, they get more complete and sticky. (GTA V quite possibly does the same, I just know less about the details there and its a lot less obvious)
Yeah, I think GTA has been doing a good job at handling these things gracefully.
Regarding your second point, I'm not sure if you've seen how it's handled in CP? Basically all roads in the distance have a string of car/headlight sprites (yes, 2D) moving on them. Makes the world look amazingly populated and alive.
However the density of the sprite cars is much higher than the actual cars (depending on your settings). So when driving on a long road you always seem to be chasing a large group of cars that you can never reach. There seems to be no connection between actual cars spawning in and the sprite based crowd illusion.
And if you use a scope with good zoom to observe the sprite cars, it looks very funny. Basically you have very obvious Doom faux-3D sprites in an otherwise stunning environment.
In earlier GTAs like San Andreas there is an explicit difference between "traffic in the distance" and "traffic close by" - the former will be removed if out of sight, doesn't have full-fidelity 3D models, apparently doesn't have physics applied to them, ... Once they get close enough, they get more complete and sticky. (GTA V quite possibly does the same, I just know less about the details there and its a lot less obvious)