Your points are fundamentally correct, and I would argue that your parent is attempting to limit the potential of ProseMirror, in order to preserve the need for Quill.
indeed, there will be demand for more/different products even after both of these competitors maximize their reach.
There use to be loads and loads of JS frameworks of dubious merit and with equally dubious developers. Now there's generally an agreed upon few (Angular 2.0, React, Vue) that are pretty solid and seem to be helmed by mostly sensible people.
Unfortunately, we're still in the early stages of open-source WYSWIG/WYSWIM;. By the later stages, I hope we can thin out the technically unsound editors with insecure authors.