You can do the same thing with RSS+We mention, which is a way simpler stack and predates ActivityPub by years
Anyway, my point was less "ActivityPub can do everything people can do with RSS" and more "having a mechanism to for bidirectional authenticated messages opens up the possibility of new applications".
The real interesting part will happen when/if more developers realize that ActivityPub can do more than "federated versions of popular social media platforms".
As far as spamming goes, I don't see how WM is any worse than AP. In both protocols your only options are passlists and/or blocklists.
[0]: And an old version that doesn't have an official spec. ActivityPub's issues with spec stagnation and de facto standards is a whole other thing.
Perhaps more advanced URL regex can achieve more fine-grained control but I do still see advantages in pubkey auth (especially if people want to move their content.)
Still, I do find myself wishing for a lighterweight-than-ActivityPub middleground.
I suspect this is because WM is used far less than AP. It also grew out of a community (IndieWeb) where having your own domain is a core tenet.
I think something like Mastodon could work with WM though, since all URLs hang off of user URLs, so you could block by URL prefix, ie "block https://example.com/user1/*".
I guess one danger is that they only serve the page that contains your link to the webmention-validating request. That way they get a backlink but don't have to keep a public outgoing link. They'd have to know that a given request is that validation though, and I'm not sure that'd be very easy.