I really don't understand this deal at all.
1. Minecraft is about five years old.
I used to know lots of people that played it. When I first started my job almost three years ago they had a Minecraft server but it never gets used now. The only person I know who still plays is a 7 yr old and even he has moved on to GTA5
Maybe if this deal had happened two years ago then I could see where MS saw value, but how can they squeeze an extra $2.5 billion from this game which everyone has already bought and played?
2. Mojang is Minecraft, and Minecraft is Notch.
Mojang are currently a one-game company, and that one game was solely created by Notch who will be leaving once this deal is done. They are basically just buying Minecraft and they don't get any of the talent that created it.
3. Why sequel?
A huge part of the success of Minecraft was it's uniqueness. The purposefully pixelated 3d world. The never-ending, procedurally generated environment that actually created some interesting shapes. The day-night cycle which forced you to mine/build during the daytime and then hide/defend at night. The UI that didn't spell out everything you can do and instead encouraged experimentation with combining items.
All of these things were created by one man and then on top of that the modding community has added everything else you can think of.
The success of Minecraft first caught everyone by surprise because the graphics were 'poor' and people were still buying it. This is not a game where graphics are important and so I can't see how the inevitable sequel will be able to improve over the original.