Depends on your POV.

In one sense, if ActivityPub can be a bridge between two protocols (e.g. RSS vs email) then it's always technically possible to cut out the middle man. In that sense, no not really.

From my POV though ActivityPub shines because it's more content agnostic. RSS is specific to feeds and posts, while email is for email, Bluesky is Bluesky (twitter), etc, but ActivityPub can handle video (peertube), images (pixelfed), forums - including likes and downvotes (Lemmy), microblogging (Mastodon), etc. (Note that the ActivityPub to email implementation I mentioned currently doesn't handle likes/downvotes for example.)

With the possible exception of email, I'd also say that ActivityPub has something these other protocols do not - ownership over your own data. If you run your own instance for yourself, you always retain a copy of your content - you don't have the situation of ello.co where if the site suddenly goes down without warning you lose years of work. Even if you use someone else's instance, if that goes down you may be able to recover your content from another instance that was federating to it (retrieving content posted to kbin.social from the copy at fedia.io for example). That's the beautify of federation.

(This is also true of traditional email, but things like gmail and Outlook - where the email is simply hosted on someone else's server - are moving away from that.)