Part of "good UI" is not having E2E, which e.g. gives you sync that actually works, even on new devices, with no weird backups and PIN codes necessary, just like the good old days.
Partly, but Signal etc. could just as well have a fast and polished client, and Telegram a clunky electron sloth. Even if you restrict yourself to one device (so no syncing) the difference in quality is undeniable.
