Yea you've seen it tried in projects like Waterfox and Palemoon and it eventually becomes too much to deal with. (Following the old Firefox addons system that is)
Yeah it's clear that was never going to work -- the whole point of dropping the old addons was making big architectural changes that weren't possible with the old APIs. You can't merge the new architectural changes and the old APIs without running into the issues they were trying to avoid by removing the old APIs in the first place.