Sure, Mastodon doesn't implement the full ActivityPub spec [0]. They're still very much behind the effort though, it seems [1] (or at least they were in 2018).Not only this, but they go against it in multiple places. How they've implemented 'summary' and the bizarre things they've done to create compatibility with Misskey (but not in an actually-compatible way, Misskey's had to patch their own solution every time) has made every implementation have to implement an against-spec trashfire.
I'm not a Mastodon developer, nor have I developed with it, but surely there are learnings there that could potentially be contributed back to the W3C to make ActivityPub better (including making it more rigorously spec'd).
Alternatively, the AP spec is fundamentally broken in many ways, and that effort would be better off going into a protocol like Secure Scuttlebutt (not a fan, but it's much better than AP) or Diaspora (also not a fan, but it's much better than AP).
The people at the W3C are not gods - they're human beings just like the rest of us. Why not get involved in the conversation there?
The only genuine reason I can think of to split away from the ActivityPub effort to build a completely different protocol would be if the W3C's working group on ActivityPub is filled with such toxic politics that it's impossible to make any progress on making the spec better. (That's been the only reason I could see in the past when I've been part of protocol standardization efforts).
Not only politics, but it's incredibly slow. Webber has written a post on how troublesome the standardization process was, though he doesn't go into just how bad it was:
https://dustycloud.org/blog/on-standards-divisions-collabora...
Without evidence of that, however, I'd say that it's far better for the community to get involved in making the existing spec better. Robust debate (supported by evidence, such as that which Mastodon's built up) is incredibly valuable to the community as a whole.
There are already existing specifications, none of them as bad as ActivityPub. I'm fully for going in one of those directions, but AP is more or less a failed experiment.
Also, it's incredibly valuable from a political decentralization perspective to have as neutral a party as possible facilitating the standardization efforts, to minimize the risk of Big Tech shaping standards in ways that would suit them financially (and potentially against the interests of the average internet user).
W3C is not neutral, and not only are they not neutral, but the RIAA and MPAA are people with enough sway to have messed up the entire web multiple times (see: making EME a standard).
A neutral party sounds great! The W3C should not get to be involved, however.
