Of course, Ardour can still be run with JACK, and this option is not going away. We hope to see these other "backends" emerge over the next few months. For now, users should not experience any functional changes when using Ardour.
I dont think this is going to effect jack at all. Jack is an incredible and highly configurable back end. Theres so much development gone into jack to get to where it is today and what it can do that i doubt anything that comes out could catch up to where jack is.
I think its just more to make it easier to develop for osx and windows since mixbus is a commercial project that uses ardour.
currently though i think ardour wont run without jack as there are no other backends yet that support the functionality of jack. as i understand it ardour cant replace what jack does, it still needs a backend that does all the behind the hood stuff.