> So the client is open source and guarantees end-to-end encryption regardless of what the server does. Ok, then I honestly don't care. Why should I?

You should because any successful centralized messenger is one update away from becoming entirely closed source.

If Signal will reach say a billion users it will be able to do that without significant userbase loss, due to network effect.

> safety characteristics, which as stated are apparently ensured by the client regardless of what the server does

In reality, the contact social graph and the frequency and pattern of messages between users is leaked.

Any global observer can do a correlation attack thanks to the centralized servers (and absence of onion routing).