I get that "trust" is the hard part, but for a circle of people that I directly know it shouldn't be hard.
I wasn't expecting the email protocol to change, just that I (my email client) would send / received cypher text (e.g., base64 encoded). The cypher text would be transmitted just like any other message.
You're right! Sending and receiving safely encoded and encrypted ciphertext is so easy and straightforward that it should be common by now.
Perhaps it could be worth considering that there could be complexities from another angle? Key discovery and management are complex tasks that require a working knowledge of PKI. These are also tasks that can be very dangerous to undertake in ignorance, as even a small mistake can lead to a full compromise. Is it possible that many users find these difficulties highly challenging?
SMIME simply included the public key with every message. Solving initial discovery could be as simple as using a well known address, e.g.: www.emailprovider.com/.well_known/kalium.pub. As for key management Apple does a good job managing this (e.g. my understanding is that Apple Messages are E2E using keys yet I've never managed on of these keys), other software vendors would need to step up.
So, it's not a user problem, but an incentives problem. Companies aren't incentivized to solve this problem. OSS could, but the network effects mean that any solution is dead unless you can get the likes of gmail on board.
You're right! Those are all excellent solutions to basic discovery and management problems.
With that said, is it perhaps worth considering that this could be a scenario where fully automating something is in fact not sufficient? As you so wisely and correctly point to, Apple has made key management work. Yet most Apple users probably wouldn't notice, or know what it meant, if they were told that whoever they were texting had a new key. From experience this is already true of messaging systems like Signal and Wire. Making available documentation that clearly and simply explains the matter and what the user could or should do has, historically, not reliably been a great way of resolving this issue.
It might be worth considering that there could be more at hand that an onerous task that just needs a smidge of standardization and a pinch of automation - though to be clear you are completely right that both are needed and would be very beneficial. Real security generally needs to involve users understanding and weighing risks. I've yet to see any method to automate that well, though I would love to be shown how I am badly mistaken.
I wasn't expecting the email protocol to change, just that I (my email client) would send / received cypher text (e.g., base64 encoded). The cypher text would be transmitted just like any other message.