What I built isn’t an ActivityPub system as much as a Mastodon-compatible one. I think this is the key contradiction of the ActivityPub system: it’s a specification broad enough to encompass many different services, but ends up being too general to be useful by itself. There are other specifications like this - things like KML which are technically open and specified but practically defined by what Google Earth supports and produces.
With this frame of mind, the question becomes, if ActivityPub probably isn’t going to be a self-contained standard and instead the basis for one or two popular, homogenous implementations, and if federation is probably going to be a secondary property of those implementations, is the specification technically good enough, useful enough, correct enough, that a future Twitter-competitor will use it?
Protocols-in-name-only are inherently unstable and also require multiple implementations. Protocols aren’t useful unless they are meaningfully specific and implemented by multiple parties.