I just can’t find a decent email client that looks like it’s from the last 20 years. Geary and Evolution both appear to be pretty modern but something about using Gmail with a Yubikey just doesn’t work and neither of them will connect to my account. Both on Fedora and OpenSUSE. Thunderbird works but it’s so old fashioned and Betterbird doesn’t look much better. What’s everyone else using?
I’ve used claws for like ten years and I have never felt any reason to switch, but OP’s criticism of thunderbird is that it’s too old-fashioned. Claws was more old-fashioned than thunderbird way back when I was trying out different clients, and has had no significant interface changes in the time since.
But yeah, claws is awesome. I can’t speak for power users, but as someone who doesn’t need a lot of features other than being somewhat idiot-proofed, it works great for me.
My work uses office365 and claws does not work with those mailboxes on its own, it took me a while to figure out the workaround. There’s a libre program called davmail that will allow you to access office365 emails from any client, it’s in the AUR and for Debian users I believe it’s in the native repositories.
TBH I’m not sure what exactly OP wants. They like Evolution and dislike Thunderbird but they both look the same to me. All mail software on desktop has list of folders, list of messages and message view.