SRV lookup for localhost names should yield a (probably identical) localhost name that (by the rest of this draft) necessarily then resolves to a loopback address.

For ports we have the list of well-known services maintained by IANA, for which an extract appears on many systems in /etc/services. Local configuration can adjust as necessary.

Local configuration cannot adjust as necessary. Remember: the headlined article is something that is being proposed to fix into an RFC, and indeed the whole point of it is to hardwire something that, in fact, is currently a matter of local configuration.