I have not stopped using Thunderbird since 2005 and while I was a bit concerned for the short period of uncertainly about Thunderbird's development, it had no practical consequences for me. I find it works well. I find no reason to stop using it, other than since I use KDE, KMail would probably be better integrated to my desktop environment but I haven't managed to switch so far. So Thunderbird's UI from the 2000's it is… but it is really efficient and KDE Breeze's GTK theme does not fail to give it a good look.