I'm not sure if this is the right analogy. AP ID is a URL under which you can find metadata and public signing keys for the account. It's an in-envelope concept (to borrow e-mail terminology) and has ~nothing to do with routing (in fact the metadata returned points at your inbox, outbox, and other collections, which can be hosted wherever the handler of that URL desires).
I do agree that it's bad that we have two levels of IDs and different pieces of software consider different levels as persistent.
> why is the domain of the identity hard-tied to a URL host?
@taral it's not really. I showed in another comment in this thread how you can have webinger acct resources with different values pointing to the same activitypub actor URL.
I think that DNS usage for resource discovery is an interesting idea, however I think having to deal with DNS would make it more difficult not less for non-developers to migrate.
@robryk @filippo But that's the thing, why is the domain of the identity hard-tied to a URL host? They could have used SRV or TXT resolution mechanisms like other protocols do.