>What’s stopping google from doing what they did with Android: remove more and more and more of the parts included with open source Android and move them into closed source Android to the point where vanilla open source Android has much lower utility?
Microsoft has more than enough resources and engineers to continue to support or even fork Blink even if Google went full closed source (just like Google did back in the day with webkit).
I wouldn't trust Microsoft to be better than Google on privacy TBH. In fact I feel that given the opportunity Microsoft has prooven time and time again they would be worse, but they aren't as successfull with Bing so they don't have the opportunity.