I’ve been using Thunderbird since 2002 and the interface has looked about the same the whole time, and worked the same. It’s a damn email client. It doesn’t need to change appearance and functionality radically every six months. It’s not (by Zeus) optimized for mobile. It just gets out of the way and works.
How people tolerate webmail, especially gmail and its horrid interface, its extreme slowness, and its data-thievin’, I have no idea at all.
I will keep using a real mail client until none exist.