Its death. Everything that Microsoft will buy will soon die. There are lots of projects that are already gone and not used by people after its being owned by MS. One that is massively used before Microsoft was skype and after they bought it, almost everyone avoided it already. There were more alternatives coming out too. Github will die and maybe another will come out and again MS will buy it.
I can nor agree on that, look at the Skype they bought it and now it doesn't even work but they bought it because people know about it and use it everyday. Same for github the only disadvantage is if they ruin some existing features or play around with terms and policies to screw everything up. They might even play with github code base and make it slower. It's like they ruin everything but have the money to do so. Github is not the only platform if MS screw it up people are gonna move away from it.