Of the extensions you listed, the largest (Classic Theme Restorer) is only used by 333,000 people while the smallest (Edgewise) is only used by 218 people.
Mozilla should find a way of moving forward while keeping the code open enough to modding. AFAIK, features offered by these extensions that I use are not implementable in WebExtensions even in principle, and that is the main problem. Break the compatibility if you must, but make it possible for someone who cares enough to step in and rewrite these extensions to the new architecture.
I'd argue that many of the users that choose Firefox over Chrome do so for the fact that Firefox is 100% open source and isn't tied to an advertising giant trying to capture tons of extra private data from you. This won't change that. It will mean you can't change the layout of the entire Firefox window and add tons of new custom buttons and things like that. But most users don't care about that.
For the niche that can't stand using a UI they can't fully customize, they're free to use a slower and less secure browser that continues to support XUL and the associated extensions while still keeping up to date with at least the slower Firefox ESR branch of renderer underneath. Pale Moon, for example.
doesn't sound like an "only" to me.
Plenty of those long-time users giving advice to the young-uns are more than happy yo sacrifice a small part of firefox for big improvements elsewhere.