It seems to me that we could just fork the app and add in any ETE encryption we want. I get that everyone is annoyed at the Telegram people, but there are a bunch of open clients and the encryption we'd want must be independent of the server. For instance I could paste in PGP encrypted messages. Maybe there is some technical reason this wouldn't work?
I think people would rather develop their own app from the ground up instead of squeaking into another app / network when all the users must download the custom app anyway.
The server sofware is closed source too, so we can't check what it does either.
What is your point then when both ends of the communication are closed source?