It’s true, email is probably unfixable. It’s ok as a digital postcard though, and sometimes that’s all you need. If we could finally get rid of either SMS or email I’d have to pick SMS. But we’re probably stuck with both due to politics and network effects.
I didn’t see any mention of Delta Chat as an attempt to secure email. I do like Delta Chat on chatmail servers (see https://chatmail.at/doc/relay/faq.html#what-is-the-differenc...). Signal is better security-wise but I am very much oriented towards federation or full decentralization. For myself, I worry more about a service being blocked than I do targeted attacks, although I understand that others have different threat models.