Not quite - as PGP is a pita to use, and not a formal part of the SMTP spec. So refusing to read non-PGP would be suicide. But if it was considered table stakes to implement the crypto option of the spec, and all the decent Matrix clients out there did so and sent end-to-end encrypted by default, then it'd naturally become the default. In other words, if you gracefully upgrade chats between capable clients to be end-to-end by default, everybody wins.