Diablo 3 would end ALOT of people's WoW subscriptions. Imagine a game where alot of the "rights" (not wrongs) are based off of WoW's success (this can be seen in previous games affecting WoW). I would MUCH rather solo or go along with 1-2 others (only when I want) to do things than be forced to go with 9 others at specific times (not when I WANT TO, but when EVERYONE is CONVENIENT).
So imagine a (albeit drastically changed and another universe, but faint reminders of WoW like talents or interfaces, or abilities) game that allows you to play and get your fix without WoW being involved.
Blizzard knows D3 would do this, so they're going to continue milking WoW. Simple business (which is sad, but they do produce amazing quality.).