The profit of a WoW subscription far outweighs 12 million box sales of any other Blizzard game dude.
Diablo3 is a perfect game to play casually and return to without the need of long term attunements.
I am pretty sure Morhaime is very pleased D3 sold massively but didn't hurt too much the long term WoW subscriptions after the dip of 9.1 million subs the months D3 launched.
Frank Pierce already expressed concerns about D3 taking too much players from WoW back in 2010.
Present day D3 play succeeded in creating a casual game you return to on a regular basis but apparently WoW survived throughout 2012 with a healthy 10 million subs, bringing in the usual 1 billion+ subscription dollars.
As such D3 fills in perfectly a gap in the BattleNet game choices. Add Blizzard All Stars and SC2 for PvP and WoW for a fantasy MMO an you will see that each game in this stable doesn't overlap the others.
Titan will be treated the same way: probably by introducing a complete other kind of on line game to fill the last gap.