I don't see why MS would want to do that but it's perfectly possible to "extinguish" GPL projects, especially something as massive as an OS.
You buy Ubuntu, put a lot of money into it, slowly force more and more of your MS software on people, make more and more third party software depend on it and eventually the effort needed to fork away and maintain "clean" version is simply too huge to be worth it.
But of course in this case there are many other Linux distributions out there already so it shouldn't be a huge cause for concern.
This is what happened with Android. Google bought it, released it, and made a big deal out of how it was open. Then they slowly moved functionality over to their proprietary systems under the guise of ensuring people could get some security updates on phones the OEMs stopped supporting.
Instead of taking away, which you can't do with GPL you can always push things that cause problems. You could make Ubuntu terrible enough that people stop using it.
2019: Microsoft bought Ubuntu