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).
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).
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.
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).
[0] - https://activitypub.rocks/implementation-report/
[1] - https://blog.joinmastodon.org/2018/06/why-activitypub-is-the...