I used to think like this, but then I realized: jj-mode.el exists[0] and you can still use magic since it's still a git repo underneath. Seriously, don't let this hold you back.
I took a look at both, and I think I'd need the jj split function (comparable to staging individual lines) to be implemented in jj-mode to match my use of magit.
You can already do this if you just open up your change in magit and repeatedly commit pieces of it. I agree it's not super ergonomic since it leaves an old change behind and it requires using magit on top of jj-mode but it's 90% of the way there.
Besides, Magit rebasing is also pretty sweet.