I've used S/MIME email within a company and on one occasion to another company. It was underpinned by a CA model. These days, obtaining a certificate for your c

I've used S/MIME email within a company and on one occasion to another company. It was underpinned by a CA model. These days, obtaining a certificate for your client is not more difficult than obtaining an app password (some providers like Gmail and O365 ask you to obtain such an app password for "legacy" or "insecure" clients). There is a definitely big margin for usability improvements. We've seen browser vendors like Chrome and Safari make such strides. I agree that the multiple device problem is a long standing unsolved problem for E2E. However, there have been minor success stories like iMessage, WhatsApp, OnePass, Signal and iCloud Keychain. These solutions do sync secrets well. They aren't for everyone because of the trust model but do provide an acceptable convenience for a significant number of people - in the same way that the web's current CA hierarchy is convenient to many.