But that's the whole problem with "trusting" microsoft. Their stated motto used to be "embrace, extend, extinguish". How do we know that has seriously changed on all levels?

I do not believe that Microsoft is fundamentally on board with the open source model. Its an alliance of convenience. If suddenly they think they can make more money some other way, they'll do that. They might not even actually destroy anything explicitly, merely ruin things out of indifference.