If you're going to say what is said under every submission about Signal you should I least mention that it is a choice they made and that they gave reasons why [1], whether you disagree with their arguments.
You can only trust E2EE if whoever controls the server does not also control the client. Otherwise they can just backdoor the client.
I wonder what'd happen if a three letter agency subpoena'd Signal and demanded Signal's app signing key so they can ship a backdoored update to a handful of targets.
Interesting how the issue there was both usage of their servers and the name.
Molly (https://github.com/mollyim) uses their servers but not their name and seems to be operating just fine. I've been using it for almost a year now.
I found it primarily because I wanted to run Signal on a tablet.
If they believe that uncontrolled 3rd-party clients would make it easier to breach their protocol (because they know their protocol's weaknesses), it would make sense to be openly hostile to 3rd-party clients.
If there's no third party client then the e2e is completely moot because Signal could be forced to push a backdoor to their customers and they would never know e2e has been removed.
How would a third party client make it easier to breach the protocol? You can still study the source of the client, the protocol, make a fork of it and modify it, etc. The official client is fully open source.
The argument for security for disallowing other clients would be being able to modify the protocol to patch out security issues without worrying about compatibility with other clients.
I am personnally glad that both options exist.
[1] https://signal.org/blog/the-ecosystem-is-moving/