This is kind of my point - I'm not saying you can't have applications that are usable across multiple UX paradigms, and I'm also not saying you can't write a UX library that automatically translates at least simple applications with little manual effort.
I'm just saying this requires active buy in from application developers into the ecosystem - you can't just run everything on all devices and have it magically work (with usability comparable to current state of the art in single device applications).
I don't think it can necessarily work for any kind of application, but for some simpler ones I think it's completely fine.