I'm sure it would be easier if they didn't have to worry about creating an environment, and I was thinking the same thing about SA. They could set up the world just like SA. Leave Tall Trees, the desert, and some farm land alone, and the rest would be city. It's weird when I play RDR and try to think about how it would look a hundred years in the future, because it's almost impossible to imagine cities with buildings as high or even higher than the ones in GTA IV, and the tiny dirt roads and paths also make it hard to imagine cars on city streets, modern bridges and highways, or towns with a lot more people in them. However, the one thing that is very easy for me to imagine is taking a dirt bike to the desert and going nuts. That would be amazing!
Btw..why haven't you played RDR, do you have a console? I remember hoping RDR would come out on PC, because I wanted to make some mods for it. Now I'm wondering if R* was trying to keep people from creating a GTA mod on that beautiful map?