FWIW it wouldn’t have been that hard to use unique identifiers rather than usernames for the technical implementation of following people. The ActivityPub designers decided on using usernames rather than unique IDs as the primary identification of a user, but they could’ve chosen something else.

It’s not possible to change your username without something like Mastodon’s move mechanism (that’s poorly supported outside of Mastodon), but the question itself isn’t that crazy. For instance, Bluesky is a federated protocol and they use unique IDs, allowing people to change their usernames in a federated system.

ActivityPub could do this as well (just let the WebFinger service link to a user ID based URL rather than a username based URL, so multiple account names could eventually redirect to the same user) but I don’t think it’s implemented that way in the popular ActivityPub servers.