And they could be more ambitious by taking Xbox Live on Windows and extracting the crucial DirectX components and virtualising them for OSX and Linux. Put that Store natively on Mac and Steam with the ability to run Windows games without needing Windows, without needing to switch to Wndows and without the less-than-optimal translation technologies (Wine, Parallels, VM Fusion etc).
This will give them a leg up on Steam on non-Windows OSs. Because you can bet your arse Valve are working on ways to get Windows software running on other platforms - particularly OSX because those people have money and spend it. With APIs cutting closer to the hardware, there will be less complex translation of fat APIs in the future. Running Windows software on non-Windows operating systems will only get easier and faster.
For that to happen Apple would need to have a huge shift in focus and either cede or share control of key parts of OSX with Microsoft, IMO.
OSX is a decent OS. However, it really sucks compared to Windows for gaming. On comparable hardware a game will run significantly worse in OSX than it will in Windows. Blizzard has been committed to releasing on both Windows and Macs for almost 20 years now and their games have never run as well in OSX as they have in Windows. Valve is in a similar place, even when they used to use OpenGL as the basis for their graphics rendering thus rendering DirectX mostly irrelevant to the discussion. There hasn't been a single port to OSX that has offered similar performance on similar hardware that I'm aware of.
Much of that was just Apple's indifference to gaming on the platform. While that has improved slightly, it's still not at a place where gaming on OSX is going to flourish.
Now assuming Apple would open things up to Microsoft to help improve the platform WRT performant gaming or more importantly being able to run the same executables and/or easily compile (basically make OSX a target for UWP, which would require massive involvement from Apple, IMO) to OSX... What would be Microsoft's incentive be for this rather massive undertaking?
Also take into account that Apple can and does at times decide to just change how things are done. Developer's can then either adapt or risk their applications no longer working. If Microsoft spent all this effort to make Windows applications (games) easily run in OSX and Apple decides to change things, then MS has to go through all that massive effort to attempt to make their back catalog work with whatever new system has caught Apple's eye. One of the things people like about OSX is how it can stay relatively fresh and new, but with that sometimes comes abandonment of backwards compatibility. One of the things people like with Windows is how it can support programs that are decades old written in the 80's and 90's, but with that come difficulty in modernizing many parts of the OS.
If Apple were serious about gaming becoming a thing on OSX, we'll see it in the results. Games and their performance. That hasn't happened yet, despite big time players like Blizzard and Valve wanting to make OSX a gaming destination on par with Windows. Microsoft won't be able to change that.
Regards,
SB