I wont speak for any of the other groups but HAVOC have not quit, a few of us do still play from time to time and even had the great fun of a 1v1 vs LEGION (a ship named Legion Drone) yesterday.
While I think that yes, Legion having an astounding monopoly on asteroids in PvP is a bad thing and will long term result in both a loss of content for Legion and ourselves I wont blame them as a coalition or as individuals. Demanding a group who have all built friendships and respect as part of a coalition to break up and become enemies for the sake of your enjoyment is a downright stupid thing to demand, I have seen these groups in almost every game I have played that is a sandbox and I don't believe it will go away any time soon.
Its a situation that is currently unworkable as it is allowing Legion pilots and the coalition as a whole to significantly catapult their financial backing through resources solidifying their position even if another group steps up, however it is also a situation in which I do not know of any solution that would solve the underlying issue without another group defeating and becoming the new Legion or failing and the status-quo remaining.
I do hope that while these things are smoothed out Legion will continue to try and limit the numbers they bring to keep fights from being too one sided. That being said being another EvE-Online refugee I accept anything I take into PvP can and will at some stage be lost so I wont bitch and whine when a few of you pop around the corner and blast me back home.