ActivityPub was explicitly designed as a successor to OStatus, which was built on Atom and Pubsubhubbub (now websub), which was a successor technology to RSS (AP also integrated lots of fundamental design concepts from pump.io, which was an intermediate step between OStatus and AP). The RDF vocabulary it uses still has roots in the same RDF vocabulary used for OStatus (ActivityStreams 1).
Though it looks like ActivityPub is push, and RSS is pull. But one should be able to construct transformers between the formats of ActivityPub and RSS/Atom.
Wouldn’t that make ActivityPub a successor technology for RSS?
EDIT: maybe not — https://macwright.com/2022/12/09/activitypub.html