Imagine if Microsoft had not been so dead set on using C# to push their operating system, but instead had spent significant resources on making C# available on Linux in a manner that wasn't upsetting to the open source community.
Rather than the "fan fiction"-variant of the platform that is Mono (which is forever condemned to be the poor-man's alternative), there could have been a viable .NET platform on Linux today -- and other Unixen.
If Microsoft hadn't seen C# and .NET as a tool for securing an audience for their OS they could have owned software development. People like to malign .NET but I don't think they don't do this because there is anything inherently bad about C# and .NET -- they do it because .NET implies Windows and Windows isn't exactly the favorite OS of open source developers. Or closed source developers for that matter.
It is easy to speculate that Java would have atrophied and died if Microsoft had targeted Unixen. It is certainly hard to imagine there'd be meaningful collaboration between Sun and Microsoft (thanks to stupid IPR laws, companies are encouraged to waste their time bickering over meaningless trivialities).
Perhaps both would have matured at an accelerated rate? Perhaps not.
In any case, Microsoft committed a strategic blunder by not ensuring they own the developers. I think might have been a mistake that is best measured in hundreds of billions of dollars.