Are we STILL talking about this? This is pretty basic business stuff.
2.7 Mil for the first week is actually better than a lot of people expected, and it's a damn impressive number all things considered. I don't think a single rational person honestly expected MoP, no matter how good it was, to outsell previous expansions. It could have been the best expansion possible and it was very unlikely to outsell previous expansions. MMOs are hard games to get into later in their lifespans, so when WoW lost all those millions of subscribers last expansion it was almost impossible to bring all of them back for MoP, and there was no way that Blizzard would make up the difference in new subscribers with the hurdles that new subscribers have to jump through. As a new subscriber it doesn't really make sense to even buy MoP yet unless you're really set on being a Monk.
Financially MoP is a success so far for Blizzard, MoP also seems to be a success for players. To see whether it's a success in the long term though we'll have to wait for quarterly reports to see if people stick around and keep playing.