I know they're quite different technically. But practically, what does ActivityPub unlock that was not previously possible with RSS and basic web tech stack?
I think I have an idea of the answer. RSS may provide a way for users to "subscribe" to content from a feed, equivalent of following and putting it in a unified feed.
But it does not have a way for users to interact with the poster, like comments or likes. This may be possible with a basic web stack though, but either users will have to make accounts on every person's site, or the site has to accept no user auth. (but this could be resolved with a identity provider standard, like disqus does)
I suppose another thing activityPub does is distribute content to multiple servers. Not sure if this is really desirable though?
"basic web tech stack" is quite a bit hand-wavy, don't you think?
Yeah, indieweb sites could do a lot of things: authn and authz, content syndication, backlinking, Social Graph, etc. But none of them had been standardized and put into one single protocol. ActivityPub is precisely this protocol.
Saying that "we can do that with a basic web stack" is not that different from saying "we can have a protocol to publish XML content without styling using a basic web stack, why do we need RSS or Atom?"
But I don't understand what you are trying to argue. Do you think that AP is only meant to be a single improvement about existing implementation for (micro)blogs?