Write once, run everywhere always has various tradeoffs. The closer you get to fundamental differences in operating systems, the more platform-specific code you need.

UI, for example, is hard to make platform independent if your application needs to look and feel native; or needs to plug into platform-specific features.

What would be useful is a way to easily call WebAssembly libraries from native applications. This way common business logic can be written cross-platform; but the parts of an application that need to remain native are left native.