As far as people continuing to play Starcraft even though there was nothing new for over a decade, the RTS genre is an entirely different beast. Starcraft continued to be played heavily thanks to being turned into an e-sport largely by Korean players. RTS games are not like MMORPGs in which you are constantly striving to progress a character and even be told an evolving story depending on the type of MMORPG it is. Without being given new heights on a consistent basis you're left with nothing to look forward to. No new progression. This is also a different period of time. There are a lot more options now then ever before for players who are tired of a game. I can almost guarantee you that if WoW stopped putting out expansions its playerbase would plummet. You would still have people around, definitely, but it would be nothing like what it is now. Blizzard will continue to put out expansions because it actually IS necessary for them to do so if they wish to keep raking in the cash on a game they already know is a success. The game is very old by MMO standards so it wouldn't stand where its at for long if nothing new was around the corner.
WoW is definitely a juggernaut of an MMO but its an aging one that is going to need constant attention to stay running unless Blizzard wants to see the bulk of its playerbase migrate to the newer models like Guild Wars 2 and The Elder Scrolls Online, assuming it's the great MMO people are expecting. Of course, I could be ENTIRELY WRONG and sit amazed as WoW continues to have millions upon millions of players even after nothing new has come out for five to ten years.