The git storage structure is not that difficult. People could implement their own compatible clients on top of it with a quite different UI. That there is nothing that overtook git as UI in popularity seems to be an indication that the interface is not as bad as many people claim.
People have added better UIs on top of git. The problem is they don't come installed by default out of the box, and unless everyone else you work with is using them too it becomes quite hard for you to communicate properly over git issues (especially in writing development workflow docs). hg has a better UI out of the box, and is notably easier for new users to pick up and become productive with.
You're underestimating how much inertia is created simply by being the out-of-the-box default, and how hard that inertia is to overcome even by better alternatives.
Not sure who "many people" are, but no one in this thread is claiming that git is unworkable, only that it is confusing. A collaboration tool like git is highly subject to network effects. The usability delta between git and a given alternative must be very, very high before people will leave git for the alternative. Ergo, git can be both awful and "good enough" to have a majority market share (although I don't think anyone is even saying git is awful in this thread).
