Thunderbird currently looks dated, and I'm glad to see it brought into the 2020s.
Sometimes current users can be dismissive of UI changes, so it's good to see fellow users who are happy. Moreover, let's not forget the new users, who were previously put off by the dated UI.
I don't think TB looks dated. It looks functional. Which is exactly what I want from a mail client. If I want something that looks modern but is dysfunctional, I'll just switch to Outlook.
* Dark mode didn't exist for a long time, and I'm glad it does, but messages are still displayed brightly.
* The list of emails always has a columns and a header; there's no simplified vertical view.
* There's a threading view, but IIRC it doesn't combine sent and received messages into the same thread view.
I had the 2-line message header view on my wishlist for a long time. Thunderbird did pretty great.
As I posted above, there is a setting for saving sent emails in the inbox along with received messages, so that threads are complete.
This.
For better or worse, I've rather come to view "modern" flashy UI aesthetics as being indicative of poorly performing software. It's not fair, I recognize, but in my personal experience, it's a reasonable heuristic.
There is a simple setting somewhere that allows you to have all sent emails be saved in the Inbox. Right now I'm not where I can look up the exact setting for this, but I'm sure you can find it without looking too long. I've been using it this way forever, all my threads are complete and easy to follow.