Once we make clear that's not just for XB, but also, and probably more, for Windows Phone, this move makes a lot more sense, except for two things:
1) The price paid, Minecraft is big, but it already peaked, so it cannot be the magic bullet that will make Windows successful on phones and tablets, however it can at least help avoiding that the gap widen.
2) Mojang, and particularly its main product Minecraft, is essentially Notch, but MS didn't buy Notch, he sold Mojang to start again from a new smaller company that put less pressure on him.
Notch hasn´t worked on the game since 2011. The Minecraft´s changed a lot since them and was released in every other platform only after 2012.
But despite the changes, it's based on his inspiration, Jeb showed he can continue his work, but his own projects up until now aren't particularly impressive. So, if MS wants to keep Minecraft as Mojang main product, everything should go fine, at least for a while, until natural saturation, but if and when MS will want to make a sequel, who knows?
Also, an essential part of Minecraft success is not being either greedy or domineering with users, giving them a lot for the little they paid, not milking them anymore after the initial purchase, and giving them as much freedom as possible. Will MS keep on behaving this way with them and resist the temptation to limit what they always received and what they can do with it or make them pay a lot more for it? Doing the wrong choice for short-sighted greed and control freakness could quickly kill Minecraft.
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