An acquisition would mean the inevitable split of support between the MS toolchain and the OS toolchain, which would eventually be left to die. Today's MS is no different than yesterday's MS.
I'm not sure that's true, but it is true that MS would have motivation to leverage Github to promote other (licensed) MS software.
At a conference years ago, a Microsoft rep told me over beers that almost everything his division did had the ultimate objective of selling more SQL Server licenses. That's why SQL Server is a required component of almost all MS enterprise software.
So you might for instance see a SQL Server backend option, and then "Github Enterprise" sold with that configuration.
This is pretty much business 101. You have loss leaders, that are solely designed to drive traffic to different products.
This is the silliest thing I've read on HN in awhile.
It appears that what goes around does, in fact, come around.