Creating a good cross-platform
GUI framework is probably harder
than creating a good cross-platform
language.
Way way way way way harder, I'd think. It's basically never been done in the modern era. All the successful ones from the past look (and work) like garbage on a modern desktop computer (I use gnucash for my dad's business, ha ha).SwiftUI is an interesting re-think, though, and as a Windows 10 user, I can tell you that the current "native" experience sucks green potatoes. There are like 9 or 12 different UI technologies I must interact with each month. All with different text sizes and massively inconsistent UI paradigms.
So I think there is an opportunity here.
But, I bet SwiftUI → Web will come first. And if that happens, SwiftUI on Windows, and desktop Linux, will be a no-brainer and will happen.
And if it doesn't happen, then probably SwiftUI won't matter outside of the Apple ecosystem, and that will mean Swift won't matter much in the end, either. (EDIT: I don't mean Apple need do this and I don't expect they will. But SwiftUI is just an API; anybody could port it to whatever environment they want, and there are already some nascent third-party efforts to implement it for HTML/web.)
Full disclosure disclaimer: former NewtonScript programmer