OSS is not some silver bullet to fix its startup problem. Microsoft needs cross platform more than it needs OSS. It needs tools that feel as lightweight as Python or node.js. It needs a platform that easily interops with everything else, instead of being limited to just Windows, IIS and SQL server.
It is about stopping people throwing away their ms stuff and rebuilding so in the future they can come back to a more lean Windows core.
If code is still in the ms stack then putting it back to Windows is an easier sell than trying to get people to rebuild.