"No privacy whatsoever" is a bit of an stretch of an argument to make, specially when talking to the general public. You have way more privacy when sending a plain text SMS or an email than when sending a physical letter with a wax seal, and most people would argue that the latter is "private" for some definition of the word.
I would even bet that any email leaks that most users would be afraid of are going to be caused by, in this order:
A) The intended email recipient leaking it
B) The email recipient/sender storing a copy of the email at rest (likely unencrypted, even if it was sent encrypted in transit).
C) An official email archiving policy from either the recipient or the sender's organization
And only at at the very bottom of the list "one of the intermediate mail relays being compromised and leaking the email in transit".
The thing with email is it's only encrypted (even in transit) sometimes. So it kinda has the same issue as iMessage. If you're a Gmail user using Gmail's web app to email another Gmail user, you get encryption in transit (and it sounds like you might be able to implement E2EE as well). But if you email dave@somerandomemailserver.com, you won't know until you send the message whether it's encrypted.
As for envelopes and wax seals--it's an interesting question. It requires a lot less technical knowledge for someone to open that envelope than to spy on messaging traffic. On the other hand, a lot more people have access to the messaging traffic and can bring scripts to bear on it at scale.
I would even bet that any email leaks that most users would be afraid of are going to be caused by, in this order:
A) The intended email recipient leaking it
B) The email recipient/sender storing a copy of the email at rest (likely unencrypted, even if it was sent encrypted in transit).
C) An official email archiving policy from either the recipient or the sender's organization
And only at at the very bottom of the list "one of the intermediate mail relays being compromised and leaking the email in transit".