I think the Open Source infrastructure of the windows eco system is not strong enough for this framework.
Look at the windows centric Delphi community.
The Linux developers are more interested in open source, windows developer are more interested in closed source
Open sourcing the core of .NET does not target the windows developers/users. It is a way to inflitrate the new embedded market that blooms with the smart phones in the front line and this is the strategic reason for this action, they also have a contract with Novel that gives them access to mono and its development which gives them access to xamarin and various embedded targets. If the open source community do give back 1% of what ms is putting in then Visual studio will be at the top of the multiplatform list for decades to come and this is an other target of theirs, they will find a way to push the market the windows again even if that means that they will open source part of that operating system. In the long run who ever betted on MS and managed to stay current is in a good spot and with this move will be in an even better one in the future.
As for the compiled code advantages they become less and less relevant every year especially with the JIT becoming more and more powerful and the fact that JITed code has the opportunity to do last minute and CPU specific enhancements that compiled does not be sure that your current advantages will be flatten in the near future at least for end user applications we still have the upper hand in embedded hardware and I think that we should focus more on that market instead of chasing after multiplatform which is a game of quantity and we can't compete.