Nothing prevents people from releasing a new RFC describing their feature and how to implement it. See: EMail attachments (RFC 1521) which came after the original EMail definition (in RFC 822). And what you describe as "email insecurity" is just a common disagreement which encryption method to use in your MIME attachment (defined in RFC 989) - your argument sounds a bit like protesting that not everyone is using Word files when sending text attachments.

(Note: EMail metadata is deeply "insecure" and can theoretically be used to glean information about communication - but if that's your concern, maybe email is just the wrong format for you and something like encrypted messages over a network of Kafka-style message streams, ideally with lots of noise in it, would be better suited).