Not a thing in here is true, especially not objectively true. As neutral as you may believe yourself, it might be a good time to step back and reexamine your priors that led you to state so confidently that there’s no usability reason to switch in particular.
The key word I used is "substantial." The usability improvements over Git are marginal and if they ever become non-marginal, they can relatively easily be added to git. This is what my comment is getting at. The only essential difference between Git and JJ is that they are different fiefdoms. There is no substantial technological difference. It's just two different social factions with marginally different opinions about how to type CLI commands.
JJ might produce commits that can be stored in Git, but the affordances are different. If Git wants to adopt them, it becomes no longer Git.
On the other hand, I'm happily using JJ while everyone I collaborate with is using Git. JJ doesn't need to "win" to be useful, it just needs to be useful enough that the people who maintain it continue to maintain it.