I'd like to see a mechanism whereby a Bitcoin node can delegate processing of unknown messages to an external process, so a P2P node can be composed out of separated programs, but such a service would be indistinguishable at the network layer from one provided by Bitcoin Core itself, so a service bit would be appropriate for those.

For instance, Insight could then offer a command set that extends the p2p protocol for doing block explorer type queries. There's no need for the protocol to be Insight specific. You'd just have NODE_INDEXED_CHAIN instead.

Having the service run on some arbitrary other port isn't particularly useful, IMO - the biggest win from having some separated protocol would be the ability to use TLS, but if you're connecting to an IP address rather than a domain name (like if you discovered via service bits/getextsrv) this doesn't add much. It boils down to minor syntax differences in how numbers are laid out in a grid. And the performance issue remains.

Additionally, nothing in this spec requires that a local bitcoind be running. What stops someone from advertising just NODE_EXTENDED_SERVICES and nothing else? I don't think a generic service advertisement mechanism is a bad thing to have, by the way, just pointing out that nothing makes this more focused than service bits already are.