> then it's irrelevant to those of us who have been lucky to find and learn sensible UXs

Only if you're a solo dev that doesn't work on a team or have to mentor new devs that haven't developed good intuitions for this.

This strikes me a lot like the C vs. safer programming language debate all over again.

When I mentor new devs, I explain to them how I use git. Sometimes I show them the workflow in magit, which makes it easier to visualize things. But mostly I just show them how their intended actions map onto the relevant CLI commands and I tell them to figure out how those map onto their porcelain of choice. I've developed this intuition thanks to magit, but I don't think magit is necessary. This approach seems preferable to me than onboarding new devs onto a new tool that is not the industry standard.

> This strikes me a lot like the C vs. safer programming language debate all over again.

I don't see how. Safer programming language address a clear problem in C, with trade-offs (sometimes arguably the trade-offs may not be worth it, and in my experience that's what the debate tends to be about). If jj is a replacement for git it should be clear what problem within git it aims at addressing. If the problem is in the UX, then to me and many others it's not worth the trouble.