What makes you think "more people" is "more enough" to offset the $7.5B + the running costs for all the new people/real estate?
There seem to be too many people thinking "MS buys studios - MS gets more gamers" is a proven concept. But is it, really? For me, I have the nagging feeling that, once again, MS had to bail out the XBox group.
Well, bailing out Xbox is the proven strategy. Whenever MS treats its devices and services as a unified system, the company thrives: if they choose to let them work themselves out, they flounder and collapse. Case in point: Windows Phone, the Zune, Mattrick era Xbox and so on.
If the main isn't in charge, those other divisions are going nowhere. The fact that Surface actually thrived unlike WP and the only difference was that Surface was treated as a mainline MS product instead of some external venture proves as much.