Here's a thought: What if the marrow treatment wasn't as permanent as the Mountain Men had thought?
It makes little difference now, but suppose Season Two had ended differently, with Mt Weather not being wiped out. If Cage had gotten further along with his plan and had been getting more of his people outside. They spend a couple weeks coming and going with no problems...until the treatment starts to wear off. With the ridiculous way the writers handled radiation poisoning, it could have come along quite seddenly, catching them off guard, after they had long enough to get overconfident about the effectiveness of their treatments. But, by then, it would be too late, since all of Mt Weather's strongest and healthiest would ends up succumbing to the instant wearing off of the marrow treatments.
Well, it's just a passing thought I had.