For the case of mail submission, there is a lot of legacy hardware/firmware out there still in use that simply doesn't support TLS, and is mission-critical, and is likely to become obsolete before anyone upgrades it. A lot of that traffic is considered nonsensitive by those who need it so trying to force them to be secure isn't going to go over well. Despite that, RFC 8314 recommends that MSPs deprecate cleartext submission and mail access, but it doesn't specify a timetable because the situations vary too widely from one provider to another.

For message relaying, your proposal might indeed work. The vast majority of inter-domain mail traffic goes through a very small number of providers. No mail provider can afford to not be able to exchange mail with gmail, or office365, or ...