What would it take for a similar thing to happen for unsigned and unencrypted e-mail? Which companies or organisations could contribute to a fast change?
E2E is another story. There's never been a good user friendly tool that allows non-experts to easily use at scale (i.e. outside of a controlled enterprise). Then there is the problem of cert distribution/discovery: How to find recipient's keys in order to send encrypted email. There is a DANE solution to this but it never caught on.
Lastly, there is the multiple device problem. People send and receive email on multiple devices (usually laptop, mobile phone and occasionally tablet). Any solution needs to account for syncing keys between devices.
The above assumes open standards based email (S/MIME and OpenPGP). some platforms have a closed solution that works on their platform but not with users on other platforms. Similar to how every medical provider in the US has a messaging portal for patients rather than rely on (insecure) email.
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.