Yeah, M2 has some attractions, but needs a much higher powered computer than RTW to play, and Empires when it comes out will probably need something faster still. So many people will end up staying with RTW until they upgrade their computers for other reasons. It also has a slightly different feel and is more naturally based around the earlier eras, so if you are interested in the earlier/Roman based time period you might choose to stay with it.

I was never involved with the first MTW, Shogun etc. but I think they still have a reasonable amount of players.