It could. Xamarin.Forms was actually inspired by Xwt, and we only later added MVVM/XAML to it.
There are various MVVM frameworks that you could get bits from to get what you want.
That said, the Xwt approach is similar to Xamarin.Forms which is to expose a single API, and that is mapped to some underlying control. For example, you create a "Xwt.Entry" and that depending on the backend creates a WPF text entry, a Gtk Entry or a Cocoa NSTextView.
It is nice in that you write the code once, and it runs everywhere.
What we found with Xamarin Studio is that this works, but our users want native experiences. This means that even if our "Entry" was native, we could not really do or express everything that is possible on each platform with a particular entry (color, font, text attribute ranges, handling non-contiguous selections, accessibility and so on).
We wanted to be able to leverage everything a native platform did.
So new Xamarin Studios are moving away from Xwt, and we are moving into an MVVM world, where bits of code are 100% shared, but we write three independent UIs for everything.