I agree that Telegram is an impressive piece of software, but it does not offer E2EE by default and the encryption model has been criticized in the past. Still, it is the best chat app in wide use anywhere in the world and this is a great achievement. Among other things, the client is open source and they don't ban unofficial clients, and they don't restrict you to a certain number of devices like some much worse apps. The channels is also a nice innovation, and Telegram has seemingly remained reasonably unwilling to comply on government encroachment (used by both sides in the Russo-Ukrainian war without too much suspicion). This is probably as well as a centralized chat app can ever do.
I wouldn't say it doesn't offer E2EE by default. It offers private chats, which are E2EE, right there and almost as easy to initiate as the regular chats.
I used Telegram in E2EE mode with someone initially, but later we decided the multi-device sync and web chat were so much more useful to us, it trumped the desirability of E2EE and we switched.
Those features together would be better of course.
Apps like Signal and Wire have shown that multi-device E2EE is possible with messages synced across. While I believe that Telegram is far ahead on a lot of features, the lack of multi-device sync for “secret chats” is mainly because Telegram hasn’t spent enough time or effort on it.
I was looking into (toying with) making a decentralized version of TDlib. Of course a lot of effort, but possible. Telegram has some of the best clients, and FOSS. Forking those with a new TDlib would be "best" of both worlds. Session did something similar with Signal clients as a base.