Well, that would be wonderful if you could keep a flock stress-free, but it's not possible. Predators cause stress. Heat and extreme cold cause stress, moving machinery causes stress, moving birds around causes stress, etc. I think blanket statements that "MG is everywhere" are harmful, in truth, because it's just an excuse to lower your guard and when you do, BLAM!, your flock gets it. My flock has gone through all sorts of stress over the years, though I certainly try to keep them comfortable. Never had a runny nose, runny eyes, etc, coughing, etc. I'd be surprised if I had a carrier of MG here. Sure, maybe I do, but they'd have to have been completely asymptomatic all along. You do the best you can to the max and hope it never happens. You just have to have a plan in place for if it does find its way onto your place. 
Keeping an MG positive closed flock is certainly one option, but it means you must always be on guard not to spread anything by going to the feed store with shoes or clothing worn around your birds, after touching one of your birds without washing up completely. Germs are invisible so it's very difficult to know if you're spreading something. That would be my only fear about keeping an MG positive flock, even a closed one. You also can never sell a bird, can't rehome extra roosters, etc. Every one you don't want must be killed on site. Lots of things to think about in this situation.