I must assume you are an opponent of privacy
Same for images inline, and the idea of attachments was an add-on. Other examples abound.
While I don't use html in my emails, it's effectively a standard now. So much so, that I have to prod some corporations to fix their stuff.
My point is, the "email was never meant" ship sailed decades ago. Change happens.
And there is no way to encrypt anything, ever, and send it to someone, who can't see it, and then copy it and send to others. A person can take a screen shot, or just take a phone and take a pic of the screen! So even if it's not simple, this problem is a non-solve, because if a human can read it, it can be copied to others.
Which means, I do not believe your criticism on this point is valid.
All encrypted communication protocols have this requirement. And all of them will in the future. By definition, you need the counterparty to be able to decrypt your message, which means you're always vulnerable to them forwarding the unencrypted message to anyone they want.
email makes this trivial.
If it was, they did us all a favour, because PGP as means of encrypting emails is a steaming pile of garbage, as it requires both, client support, and the counterparty to have the same OPSEC as you (e.g. not just forwarding the email unencrypted to someone else)
Email was never meant to be encrypted, and the existing implementations (including S/MIME) suck for this exact reason. And the worst thing is that itβs simply not possible to make it work.