If so much mail is sent between the sponsoring organisations that the aggregate hit rate will be approximately 100%, then wouldn't approximately 100% of mail be sent over the TLS only port were the sponsoring organisations to decide to only use TLS among themselves (e.g. a hardcoded SMTPS everywhere list) ?

The point of an open standard is so that the vanishingly small fraction of people running their own mail servers can easily implement it too.