It's worse to use an encrypted but potentially compromised channel than a plain text one only when you assume the encryption is not in fact compromised, which you shouldn't do.

So I fail to see the problem. It still likely protects you from nosy neighbors or nasty non tech savvy competitors, even if it doesn't protect you from state level actors or from Facebook itself.

The above, in addition to the fact that open source is neither required for auditing, nor guarantees proper auditing occurring (see OpenSSL having vulnerabilities for years before anybody released them to the public)

WhatsApp uses a proprietary (fork of XMPP) messaging protocol and does not interoperate with anyone else (intentionally), like Hangouts and Skype, and unlike Matrix which is federated, and has gateways for IRC and XMPP already available.

> Once a client recognizes a contact as being fully e2e capable, it will not permit transmitting plaintext to that contact

They do this to prevent degradation attacks.

Once your number had e2e support0, you can't switch to a phone without e2e. Notifications on key changes are switched off by default, but I hope they still warn you in the chat history. We'll see.