why does Git need to be replaced?
replies(2):
It's a relatively easy thing to adopt something you don't need, when you are the only person involved. But VCS is pretty much in the same category as instant messaging and social network platforms are. In a sense, maybe it is a social network platform. I don't see it happening and I highly doubt I would want it to happed, even if I knew this jj stuff better.
The nice (and quite unique) thing about jj is that it takes all the good cues from systems which kept improving (and in particular, from mercurial), and slaps them onto git without requiring a storage model change, practically offering the social "perks" of GitHub (by being fully compatible with it), with no-compromise. And to be honest, at this point, there isn't much reason for a new comer to learn git rather than jj, and that's a wonderful news in my book.
The reason would be that jj uses git as a backend, and all your coworkers use git, and the whole world uses git, so using jj while not knowing git is like being "html-programmer" that doesn't know what RAM is. Surely one can live like that, but I wouldn't want to work with him.
On other hand, what is the reason to learn jj rather than git? What actual problem it solves?
"git is the underlying protocol that jj is compatible with, and sites like GitHub speak git's protocol", there now you know what git is.
We previously saw that git is in fact two very distinct things: a repo format and a user interface; and we saw that git is too critical a mass to be dislodged as a repository format. The next best thing we can do is then to address its disastrous UX, and this is precisely what jj is about. And it tackles that without even incurring "contagious" changes in established organizations: neither you or I have to know/care whether others are using git-cli/vscode gui/jj/whatever.
This is progress, because teaching git to this day is in equal parts teaching "DVCS theory", git "the useful parts", git "the very many pointy bits not to get too close to", and git "I messed it up, please help me rescue my files". At least jj offers to simplify the learning while improving on the experience greatly, by offering a very cohesive, straightforward, discoverable safe and mostly trouble-free implementation whose tenets that can be intuited from the theory.