Let's take a second to remember that Pavel Durov is the CEO of a messaging company that doesn't end-to-end encrypt chats by default and does not provide a way to end-to-end encrypt group chats
You probably don't realize it, but multi-device support, group chats of tens of thousands of users, channels with millions of users and comments of said users. Attachments and history and search. And then a whole infrastructure for running bots and processing payments. And proxies to fight blocking attempts. All of these are either highly problematic or computationally intensive and practically infeasible with E2E on.
Otherwise, someone would have taken the opportunity and reimplemented Telegram on top of homomorphic encryption /s.
Small groups could likewise be supported.
I take the point that for large groups client fan out of the double ratchet doesn't scale. We now have MLS but we didn't. There's also an argument of how can you really keep a secret in an open group of a few thousand people.
But on a small, "personal" level, e2e could absolutely be done. Not doing so is a choice, one that conveniently ends with almost everyone's chats stored on servers somewhere.
I would go so far as to say I bet telegram is a goldmine for TLAs.