Where else do SMS remain common?
At least here in Europe they've been relegated to automatic communications, like receiving 2-factor codes. No one uses them for messaging. I think the app of choice is different per country, but I haven't met any non American that's still used to SMS.
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".
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.
Personally, I hate iMessage because, while I have an iPhone, everything else is Linux. But that's not really an argument I can make to literally every single person I ever interact with. :/
It pained me not to be able to recommend iMessage but because it's not supported on Android, Windows, or Linux, you fall back to SMS (for Android) or nothing at all (for Linux/Windows) and then it's far worse than Telegram.