Well, firstly, Apple didn't have anything to do with Windows support (neither did Microsoft AFAIK).
Secondly, I don't think Apple considers the language itself to be a part of their "special sauce". There are lots of reasons why it makes sense to open the language itself up, so that it can be more widely supported on server platforms, for example. I doubt even Apple uses macOS as a server.
Of course, it may still happen. Anything might happen. I'm just saying that this is pretty wishful thinking. Not only is there no evidence to support it, but there is arguably evidence against it. Lots of people have been wishing for years that they'd open-source UIKit. And did they?
I don't think Apple just forgot to mention they'd be open-sourcing SwiftUI. If there were any such plans, they would have said that.
"everyone benefits" except Apple and their customers. Suddenly they're just getting ports of Android Apps instead of the bespoke experiences they've been used to.
At the same time, "bespoke experiences" shouldn't mean "difficult to create" or "requires expert knowledge". The goal has always been to make it as easy as possible to create great Apps which behave in a way that is natural to the platform.