I would dispute your contention that 'most' ISPs support (opportunistic) TLS for SMTP. Looking at the email I receive, both at gmail and self hosted domains, almost no one will do TLS - only big internet companies like eBay. No clients that I'm aware of maintain a history of opportunistic TLS success.

Maybe Let's Encrypt will change TLS adoption but I doubt it. It would need Google/Hotmail/Yahoo to increase the spam score for non-encrypted mail. But it's still way too complicated.