The issue here is that games and commercial productivity software are the two main reasons people(power users) still use Windows. If Proton/wine begin to seriously threaten the market share of Windows (say loss of 5-10%), they'll have to do it, I think.
They'll lose gaming lock-in, that's already inevitable. Their only remaining leverage now is Xbox, and even there translation layers can undermine their API lock-in grip.
However attacking FOSS projects using such copyright trolling will backfire and very strongly. In doing that, MS will admit their hypocrisy and all this "MS has changed" will be proven completely wrong.
I'm not saying it's not possible for them to do such attack. But it will basically completely ruin the image they are trying to build now.
