I think that's mostly a function of handling identification of the sending party. iMessage tied to an AppleID can still be controlled pretty well when it comes

I think that's mostly a function of handling identification of the sending party. iMessage tied to an AppleID can still be controlled pretty well when it comes to spam even if it would use a completely open protocol and was open sourced. Similarly if you'd only accept cryptographically signed mail by default the spam problem would go away as a bye product.