I'm afraid that would make the "protocol-level" stuff be assumed to be mandatory by default, that would cause the protocol to get bloated very easily. I've merged NIP-15 into NIP-01 to make it clear that NIP-01 is the mandatory "protocol-level" thing and nothing else, should probably also merge NIP-20. Aside from these, there is hardly any other NIP that is "protocol-level", I think.