Earlier quoted context omitted.
> they'd have pushed for replacement of XUL APIs in WebExtensions in a non-blocking way Which "they" did, and are. Some APIs are more amenable to such replacement than others. For example, APIs that allow you to modify any part of the browser UI in an arbitrary way are not so amenable _and_ have the problem of breaking any time the internal structure of the UI changes. Which has, in the past, prevented improvements t…
Except the downsides are mostly for the users (extensions stop working) while the upsides are mostly for the browser developers. (Better ability to refactor and update)
The upsides are just somewhat more diffuse than the downsides: the former apply to all users, while the latter only to those who have extensions that stop working.