they don't do it because they can't or they don't want to or they're scared or they don't think they should or their friends tell them not to or...

They have their reasons, and it works for them. We LBSs generally don't agree with their reasons, just as they don't agree with ours. To each our own.

It'll be sad, and we'll probably feel bad leaving somebody behind that we worry isn't healthy enough to really deal with life.

It is sad... but at the point you can say "bye" you may also be at a point where you figure they've made the best decision they could make for themselves, regardless of its health, and not be concerned about it, though you may still care about them. Does that make any sense?

he destroyed a good M but I'm the one doing the work and he sits back and shakes his head in total denial that the M will survive. He's just not sure and I don't think I have the skills to reassure him. Anyone have any tips on skills to reassure H that our M will survive?

He shakes his head because he's not convinced yet. You've only just put away the anger. Only consistency in the changes and being lovingly and caring there will have a chance at the desired outcome. Be patient. Learn the new dynamics of handling your relationship and put them into practice. Show by your actions that you believe your M will work.