I think this is good healthy criticism, but I think it's also difficult to strike the "right" balance.

I believe we got Signal end-to-end encryption in all of these messengers just barely, even as "compromised" as you may think it is. Google and Facebook (Messenger) didn't even enable it by default because they thought it was "too much" encryption.

So if it was even more difficult to use, it may have never been adopted by these services.

At the same time, I don't think we should allow all sorts of modifications to the protocol and to how this encryption system works just to cover a few niche use cases that would slightly increase those users' convenience.

Sending undelivered messages when the recipient is switching SIM cards instead of just telling the sender that those messages can't be sent then is one of those niche use cases and compromises that shouldn't happen, especially if enabling such features could be turned into defacto "legal intercept".

I guess Moxie is saying here that this wouldn't be a defacto legal intercept, but I'm not so sure that's true, and the researcher that found the bug doesn't seem to agree either. I think, unlike others here, it's very likely that people wouldn't notice that the messages don't have a double check mark anymore.