Unfortunately, I don’t have any ideas or solutions for you, but I do agree. I typically don’t use threaded conversations, mostly because I relied on the Conversations extension (not yet available on 115). I also wish threaded conversations had some other user-friendly behaviors: highlighted threads with new messages exactly like any other new message (right now it only semi-highlights/underlines threads with new messages), and the ability to have an expanded thread collapse automatically when opening another thread.
I also agree on the tab bar as well. I get turned around often with the placement of certain icons.
My understanding is that 115 is just the first step towards the “Supernova” experience, so hopefully a lot of these changes will be implemented as time goes by. They are basically reconstructing things from the ground up. Thankfully 102 should continue to work great.