Indeed, if you provide an API you'd better commit to it. The difference is that before WebExtensions, there was no API. Everybody was just hooking into the internals.
That's not "do not break userspace" – which makes sense – that's "do not change anything, ever" – which is project suicide.
Now, with WebExtensions, there is a difference between the API and the internals, so we can commit to something. And, while there is a cost to this change, that's definitely a much, much, much better base for developers on both side of the API.
Mozilla pitched the Add-on SDK to developers as that kind of API. It was deprecated less than a year ago with no migration path and a half-baked replacement.
That's not "do not break userspace" – which makes sense – that's "do not change anything, ever" – which is project suicide.
Now, with WebExtensions, there is a difference between the API and the internals, so we can commit to something. And, while there is a cost to this change, that's definitely a much, much, much better base for developers on both side of the API.