While I agree with your and a more open/private telegram would be great the thing is that the current alternatives, Signal, Matrix, whatever are simply subpar alternatives when it comes to the clients. We can say a lot about telegram’s privacy but the true is that all their native mobile and desktop clients outperform everything else out there. (I wonder why a native app would outperform react garbage :P).
The only think that might be private yet as performant as Telegram might be some kind of XMPP but the issue with that is that mobile clients suck.