I can see using a "pkcs7-" prefix for anything that uses pkcs7, and
I understand that micalgs have sometimes been specific to a particular
protocol. But I can't see any reason to require that all future
S/MIME micalgs use pkcs7.
It would seem far more desirable to move toward protocol-independent
micalgs, so that a message can be signed with a single micalg
but using more than one signature protocol. If I could sign the
same message with both PGP and S/MIME, then a lot more people could
verify that I signed it!