E-mail is a lingua franca. It's used not because it's superior, but because you don't have to worry about whether your recipient is using the right software setup to receive your message. It's the lowest common denominator of internet messaging and can only be replaced in that role by a new lowest common denominator.
- A company that rejected basic email would necessarily be rejecting some percent of legitimate messages and/or increase their IT costs. While this doesn't mean it's impossible, it would be at least be a painful transition. Users will hate it.
- Adding PKI just amplifies the software setup problem because now you have to worry about primitive selection, centralized authorities, key lifecycle management, etc. And there's no way for the sender and recipient to negotiate security parameters, so they have to be agreed on in advance, something basic email doesn't need.
- PKI is too finicky and abstract for the average user to understand or care about. We can't reasonably expect them to make good decisions about a subject that even professionals and large organizations struggle to understand. A big reason for email's longevity and success is that the average user doesn't need to understand it at any technical level.
Not even his supporters are going to remember him as anything but a penis butler for Trump.