One could also ask, does UO need more and more expansions?
Does the fact an "other" game has had more expansions, mean the core game is better or worse?
My personaly feeling is that a better game, draws people based on its core merits. If the core merits are good enough to retain/attract players, then it doesn't need many expansions. Just the occasional introduction of new foes or the continued evolution of any story lines. Or alternatively, continued improvements to the user interface or client, with the aim of improving the playing experience.
Sadly though, it's my opinion that since possibly Renaissance and undoubtedly since Age of Shadows, UO headed down the path of "need" for expansions, simply based upon the changes in game mechanics.