he thought it would be better for the user experience
Is this articulated somewhere because I was under the impression that everything was federated, and this plays right into the point. Why should this be up to the devs? Or, perhaps better worded, what information does the "ActivityPub" label actually tell an end user, right now? Seemingly nothing at all, from a functional standpoint. It's possible for two ActivityPub-labeled implementations to be completely incompatible, right? Does that sound good for users?
I just can't think of a devastating real world example.
Why is this your chosen metric? Wouldn't "this might make the users confused" be a better metric?
The extinguish step is a bit unclear to me.
Once they're the de facto standard they abandon it altogether and the users, who care little about the nuts and bolts of this, get frustrated and make an account on Threads (using your example).
It's worth keeping in mind that we're not talking about normal software. A hypothetical technically perfect solution is still a failure if there isn't a critical mass of users to make it "social".