The problem is that in the open source world, most projects use Git and most active old projects using SVN or CVS are slowly moving to Git. Many people out there who greatly prefer Mercurial to Git admit to using Git much more than Mercurial due to the ecosystem.
>git won the mindshare war. I regret this - I would have preferred Mercurial, but it too is not looking real healthy these days. I have made my peace with git's victory and switched. I urge the Emacs project to do likewise.
That's fine. Take Evolve as an an example of how git itself could improve. I'm fine with Mercurial being an experimental display of features that could go into git.
Also, this whole popularity argument, if it were to be believed, could have killed Linux in favour of Windows or FreeBSD in favour of Linux. Just because something is popular doesn't mean that alternatives need to cease development or are not worthy of attention.
This thread started with a request for exciting new features. A certain degree of experimentality and non-popularity is in order.
I think the distinction is that Linux is very different from Windows in a lot of important ways, while Git and Mercurial are relatively similar in what they do and how they work. If someone invented a VCS that completely revolutionized the process (I really don't know how it could, but just hypothetically), then it could serve as true competition to Git.
This is one reason why D has failed to really take off. Both D and Rust are looking to essentially take C++'s place. D is C++ improved, while Rust changes the whole model of managing memory and also introduces some new paradigms (more type inference, pattern matching, etc.). D is clearly better than C++, but the two are just similar enough to make C++ a more preferable choice due to its immense popularity compared to D. Rust changes just enough to give it a chance at being a serious contender: the value may be higher than the cost of leaving C++ for many people.
The same applies to Git (C++) and Mercurial (D). Perhaps even more so, because while Git isn't perfect it generally "just works" for almost all use cases; Mercurial is just some simplicity and elegance on top of some workflows (which are sometimes emulated with wrappers over Git, like Legit). C++ on the other hand has a lot of warts.
See: https://fuzz.me.uk/git-vs-hg/
From Eric Raymond:
>git won the mindshare war. I regret this - I would have preferred Mercurial, but it too is not looking real healthy these days. I have made my peace with git's victory and switched. I urge the Emacs project to do likewise.