I agree entirely. You'd still have a string namespace, but that string namespace is more easily subdivided.

One key downside, though: for a conversation with any service not in your cache, you need to resolve the service before connecting, rather than knowing in advance. So there's still an advantage to any service that can use a well-known port. That advantage partly goes away if you can resolve a hostname in the same DNS request as a service on that host, but that would require improving the current level of integration between clients and system resolvers.