The bigness is part of what DOES keep players, IMO. Every time you think you've got everything covered, you discover another layer to explore, dissect, analyze, and master.
One additional point: I don't think we need to worry about attracting new players or retaining old players, other than periodically adding new powers, maps, costumes, stories. Many of us have been playing this game for close to 20 years.
I may occasionally get bored and play less, but I always come back. I don't think I'll leave as long as I can see a screen and push buttons.