When would you even want to target X11/Wayland directly? Just use a toolkit and ignore the underlying stuff. For 90% of the application it does not matter.
> I find it difficult to see why this is going to change any time soon, the performance of a well written UI in a modern browser out-performs a native desktop application in every category of sped, usability and presentation for most usual use cases.
I always encounter the exact opposite. I've never found any web application with a good UI or any web application that outperformed a native one.
> Could you imagine a future where desktop applications were built like this, and in every sense of the word, “portable between operating systems” with the experience being identical on all platforms…
This sounds more like a really crappy future, if anything looks everywhere the same. Applications must integrate with the "host"-OS.
The only reason the web stack is used for desktop applications is because it is a bit cheaper and less effort than writing a good - native application.
The lock-in argument is just a very weak one. At some point you depend on something. Oh, your HTML only renders fine in Chromium or you use Chromium only JS APIs - you are locked-in to Chromium now.