While I understand what you're getting, bear in mind that in the context of a discussion about Thunderbird specifically and not just in general that Thunderbird is currently best described as being built on "A now-very-deprecated vaguely web-esque technology stack with no future". Moving to web technologies is a step forward even if you don't personally like them much. (And I'm at least sympathetic to that viewpoint.)

What we really need is a good way to execute desktop apps as if they were web services: accessible from nearly any device, access to your data with minimal complication wherever you happen to sign in from, and a lightweight, instant-response, zero-install interaction model. If we value "native clients", we need to address the things that make web clients more convenient.