This is definitely a big and newsworthy announcement from MS Build. And one of the few times that linux was given any respect as a platform. I think it still remains to be seen as to how serious MS is about real cross-platform products and work, but it's a great step. If they make their cross platform products roughly equal, not like that mono crap, then I would consider that to be a real effort by MS.
This is to stay relevant and that effort won't change... of course they are a company and may not offer a lot of energy if there isn't uptake. My limited use of the new editor is actually a pretty positive so far. I'm going to give it a try for a week, already I like the UX more than sublimetext.