The linux user-space ABI is extremely stable. I think the one thing that would frustrate the development of long service lifetime software on linux would just be library availability on various distros, but the actual operating environment presented by the kernel and the image loader for user-space software on linux is very, very stable.
And all future Win32 implementations must be 100% binary compatible with previous ones
And no one is asking microsoft to break the windows user-space ABI. Adding a new, sufficiently-tested, MBCS codepage would have no impact at all on existing windows software. None whatsoever. Other than to make localization a whole lot easier. And compared to the cost of building in a whole new csrss-level subsystem like the one Microsoft just built in to windows last month (linux), it's probably a lot safer and easier to test.
Working in this world myself, windows developers become philosophical when discussing localization. "Someday, we'll turn on UNICODE," "We really should be using TCHAR" (as if that silly thing would fix anything at all), "Shouldn't we really be using wstring?"
OSX, Linux, and mobile developers just do it. It's mostly a solved problem on those platforms.
The decision to stick with 16-bit Unicode is an engineering tradeoff.
It's a cost tradeoff - Microsoft doesn't want to spend the developer and testing time adding a UTF-8 codepage.
Currently we do "support" CP65001 in the console, but things break if you enable it. One of the problems, for example, is that .NET sees 65001 and starts outputting the UTF-8 BOM everywhere, breaking applications that don't even care about the character encoding. I suspect that's going to be difficult to fix without breaking compatibility.
Having said that, I think it's apparent that we are investing heavily in the console for the first time in a long while, so I'm more hopeful than ever that we can get this fixed.
The guarantees that Windows makes about backwards-compatibility go far beyond just the image-loader and syscalls. For comparison, the glibc backwards-compatibility story is... messier. https://www.kernel.org/pub/software/libs/glibc/hjl/compat/