There is no problem with what Ghost is setting out to do. For some of us WordPress is not reacting quickly enough to adopting the 21 century way of providing user experience. Obviously it's unrealistic for WordPress itself to drop everything and focus on this (that just isn't how life works), but it's certainly interesting and worthwhile to explore how modernisation of WordPress will enhance its original reason to exist. Looking at Ghost from this perspective and all the other reasons that John mentions in a comment below makes forking WordPress the right thing to do
This project doesn't need someone who loves WordPress and wants make more plugins for it. It needs someone who thinks they can make a good thing even better. Steve Jobs didn't just make a new cover for mobile phones. He fundamentally proved that communicational devices must become many times better than what they were before and that's what I expect from John.
Even Steve couldn't fix AT&T so don't expect John to fix WordPress core issues. That job is for WordPress itself.