Here's the situation: people hem and haw about Telegram's cryptography, but what we should really be talking about is that Telegram is not using end to end encryption by default. Telegram stores your entire plaintext message history server-side. There is nothing worse when it comes to privacy, but it's very easy to write slick clients when they're just views onto the server and all the logic happens there.
I'm pretty sure that you are aware that UX is a problem. I'm saying this because I do think UX is a priority for users and hence for widespread adoption.
There are legitimate reasons why Telegram is more popular and as evidenced, security first is not a sufficiently valid sell for Joe User. Obviously, there are also fundamental reasons why Telegram should not be used as you have pointed out above.
I want to see Signal succeed which is why I've made the comments about.
While I have your attention, maybe you could clarify the protocols used between Signal and WhatsApp. Is there a scenario which there will be interoperability? I believe that you previously alluded to the implementation of e2e being separate in WhatsApp but it would be nice to get some clarity around this. Thanks.