That would be a huge mistake if we expect many more hardware and OS running them efficiently than just iOS and Android. Best is to have an IR. Now maybe that IR can be turnt into native code. But we shouldn't be constrained. As an incoming framework author, SwiftUI is problematic for instance because it has a programming model which is dated. And I don't particularly enjoy the language either. Looked fine at first and then got more complex than I feel is needed.

oh yeah, disclaimer: I write UI frameworks and dabble in PLs.