Neovim is an effort to modernize and remove cruft from vim, so they get to keep all the good parts and throw out the backward compatibility. If it works out it can eventually replace vim, not to different to what vim did to vi.
I'd like to see similar stuff done to much of the GNU tools. Make for instance has to worry about backward compatibility and posix compliance that makes it hard to progress. As of today there have been about 12,000 attempts to replace it with something else and I find all of them inferior for one reason or another, they've all reinvented the wheel poorly. If someone had taken the fork and modernize approach we might have something better by now.
It doesn't even have to be a "hostile" fork. The same can be done by the developers of the existing tools.