That's kind of the essence of an MMO in a way,.
well, aside the "massive multiplayer", "online" and well "roleplaying game" stuff,. obviously.
But how do you keep that many people interested, online, and playing the game?
Sure they could keep busy with each other - but if you focus squarely on the "roleplaying" part,. that's not going to work. It'd just be a big MUD (multi user dungeon),. with no content.
Content keeps people busy,. and timesinks are content which keep people busy a LOT,. it's just a logical conclusion in the end.
The trick is to disguise the timesink in such a way that people willingly perform the task, again, and again in different forms.
Blizzard has done a great job at it over the years. Revolutionized the market even.