I think Minecraft has "legs" and will be around for a while. Longer than Farmville 2, less than Legos.
But I do think it says something about Microsoft. They are having a hard time growing organically, which is the curse of many large mature companies. These companies tend to expand by buyouts and mergers, which we are seeing here. Buyouts and mergers have a poor history of returns on investments.
I think Microsoft is trying for a single or double and not a home run. Maybe a 25% return over 5 years.