With current trends in Microsoft's Developer Division, it's possible that GitHub could end up way more open if Microsoft acquired them. GitHub's core business is GitHub and they need that to stay closed to stay competitive. GitHub under Microsoft could have less financial pressure to keep closed.

Is it likely? I don't know. It is an interesting possibility though, that would certainly change the game.

There is precedent. Prior to Microsoft acquiring them, Xamarin was closed-source and licensing was $1000 per platform per year. At the Build conference in March 2016 Microsoft announced (1) that they were acquiring Xamarin, (2) that pricing would be dropping to zero, and (3) the Xamarin platform code would be open sourced as soon as possible after the acquisition.

Yeah, for dev tools Microsoft was always very open. Github and Microsoft might make a great combo, boosting dev productivity and open source

They already do: Visual Studio Code is based on GitHub's Electron shell. :-)

That precedent is even more useful as a measuring stick with Nat Friedman (formerly CEO of Xamarin) announced to takeover the CEO role at GitHub.

Yep, and vaccines are bad for you and the earth is flat...