There are a million Fediverses. ActivityPub is already big and diverse with many different extensions and applications.
Nobody has to push their ActivityPub profiles or extensions to the WG if they don't want. Anyone can make extensions to ActivityPub under whatever license they want.
The onus is on extension creators to make their work compatible on the wire with ActivityPub core.
Nobody who works on ActivityPub core wants to break well-defined extensions on the Fediverse.