I used to use hose 90s GUI tools - VB's, VC's, Symantec's and Borland's java gui tools. Although they did work well for fixed UIs (absolute positioning) -- they were rather hard to get non-fixed uis working. IMHO Bootstrap's grid system was a real leap in this regard and (to me) still a pleasure to work with.
The main reason WASM and whatnot might make it easier is that instead of depending on thousands of Javascript libraries the way that so many modern web apps do perhaps a more controlled environment might include a better set of standard libraries and controls.
Also Javascript has evolved at a lightning pace which is remarkable but also makes any complex tooling for it tricky. 10 years ago jquery was the main thing, today React,Vue,Angular and so many other things are front end tools of choice.
But all that may be totally wishful thinking.
But yes, it wouldn't support every language that could compile to WASM and you're right that would be even harder than creating a good tool for Javascript.
And while it's technically possible to say, just compile QT to wasm and give it a CANVAS tag as a dumb drawing surface, you've just broken accessibility, and that's a deal breaker.
By the way, Qt-on-wasm-on-canvas is already a thing: http://blog.qt.io/blog/2018/05/22/qt-for-webassembly/
And while accessibility is a big issue, I must sadly acknowledge that I am yet to have a project require compliance and validate its implementation, as such it never gets done.