The point is simply for them not to be competitors any more.
I think they're actually buying their way into competition.
Though there are rumors that Microsoft will sell off the Xbox portion, they're still running the race. The Xbone is floundering hard, and Microsoft wants to diversify. A big thing right now are open-ended creative/interaction games, where you don't necessarily play towards a goal or endgame, you just play. Minecraft is at the fore-front of this, but it's on everything. Sony has Little Big Planet, Nintendo has Animal Crossing (and, to a much lesser extent, Tomodachi Life), the PC has Garry's Mod and probably a whole crap ton of stuff I'm forgetting. Microsoft has shown off Project Spark, but that's still in the nebulous future and doesn't seem to have much hype surrounding it.
Minecraft is now. with lots of hype. While they won't be pulling it from other devices, they can halt all new porting and put Mojang to work on a sequel that they have complete control over and release only for the Xbone (and maybe Win9 if they want to pull a Halo 2 again). Even if they don't try to do Minecraft 2, they still get all the merchandising, new DLC (which happens to be distributed only to Xbone...), and revenue from sales to other devices.