I'm not sure that's a given. Lots of things have reached worldwide adoption and don't have the reputation that Git has. Bash, for example, is quirks and chaos galore, and I don't think even that has the same level of infamy that Git has. In fact, if we use the metric of "snarky XKCD cartoons about this topic", which is probably a good proxy for how popular a tool is vs how many serious problems it creates, Git is up there with Python packaging and regular expressions.

You're right that all tools have their quirks — jj is surely no exception. But it seems like bad engineering and a lack of curiosity to look at the quirks in a tool and not ask if we can do better. How much of a tool's difficulty represents the underlying complexity of the problem it's solving, and how much is just incidental complexity from the tool itself?

In this case, I think the core of Git (the underlying model of commits and refs and objects) does a really good job of being exactly what's necessary and no more, but the UI of Git (i.e. all the commands and surrounding paraphernalia) has a lot of incidental complexity. That is, there are simpler and cleaner ways to represent a lot of the stuff that's happening in Git. This, I think, is why a lot of people struggle with Git — not just because version control is hard (although it is), but because Git as a tool is additionally hard on top of that. And I think it's reasonable to look for better ways of doing version control that don't have as much incidental complexity.