Ive been wondering the same thing. especially when u see what can be done with WASM in apps like Figma. I believe most of the QGIS code base is c++ so i wonder if it's just a case writing a bunch of js, wasm glue code to make this a reality...
Next would be GRASS.
JSTS is pure JS and does a lot of vector GIS fine.
I think those three things and a decent UI would be an absolutely killer Web app for the general public to do more GIS.