What happened 4 years ago with Firefox 57 was different - the old architecture was not sustainable (compatibility / security nightmare) and migration to WebExtension API was a necessary step.
And look where it got us - you have now the same API in Firefox, Chrome and even Safari and Thunderbird - this is amazing for me as a developer because I can release same extension on 4 distinct products with minimum changes
What we see now with V3 is a “small” evolution step - update - that is suppose to improve things. And many extensions won’t be affected at all.
But I totally agree with you that the persistent background page makes things much easier for developers.
Actually, I’ve been thinking now about my extensions and I have to say that this is gonna make a lot of problems for me . My in-memory storage with encryption keys won’t work, my canvas probably too, sub-minute timers (since Alarms have minute granularity), and all those cached data… oh man, I hope this will take a few years for them to finish .