Well, if we take a look at the IV-V transition, people in large part went on to the new game. And why not? The publisher enticed them with DLCs, better graphics, new and improved Creation capabilities, better online where they don't have to avoid using certain moves (lag tactics and stuff)... scII hd online was a step back not because it went back to a classic from a bygone age, but because its online was a major disappointment. How many people on your friend list are playing that game nowadays...?
My point being, it's only natural that people migrate to a new game if it provides just enough satisfaction.