They did add that. With the initial Windows 7 release. In 2009.
I have a hard time doing that, Wine is already so good. I can certainly imagine better but even 2x better is kinda hard to picture.
The decor and fonts look like Windows 95. Unicode characters don't display and show as boxes. Input methods don't work. Lots of hardware doesn't work. There's no obvious way to configure all the things you can usually configure in Windows. GPU accelerations don't work properly in things like Adobe's products.
It has come very far but only if you judge it by the fact that everyone is doing work for it for free.
Actually, there is a company called CodeWeavers[1], which sells software based on Wine. CodeWeavers's Wiki page says "The CrossOver version of Wine is regularly refreshed with the latest free Wine patches; likewise, patches from the company are sent back to the Wine project almost immediately. CodeWeavers is a major contributor to the Wine project ... hosts the project's website, and employs the project's maintainer, Alexandre Julliard, as their CTO."
Alexandre Julliard has his own Wikipedia page, also saying "He now works full-time on Wine for CodeWeavers."[2] If you look at Wine's changelogs like in https://www.winehq.org/announce/5.19 , you see his name on a bunch of commits. Piotr and Jacek Caban are names I also recognize as having been prominent contributors for years, and googling quickly shows they are CodeWeavers employees as well. So it seems true. I'm tempted to go buy CrossOver just to support their effort.
That is basically what WSL1 is: WINE, but backwards.
They wouldn't necessarily need to sacrifice backwards compatibility. They would just have to ensure that the new OS fully supports all of their APIs from the past.
Imagine Wine but 10 times better, built into the OS natively, and built by 500 engineers with full-time pay. I'm sure it could be pulled off, if they wanted to.
But they probably don't want to.