That mutability is what Firefox XUL offered to extensions for many years, and when it was taken away there were — and still are — cries of outrage from devoted

That mutability is what Firefox XUL offered to extensions for many years, and when it was taken away there were — and still are — cries of outrage from devoted extensions users who expected to be able to modify anything. So the model has lots of downsides, but certainly it’s not a dealbreaker.