Though not likely, a collapse and civil war would be disastrous for both Koreas.
A North Korean invasion of South Korea would be a disaster.
A North Korean sustained barrage of Seoul would be a disaster.
A North Korean use of nuclear weapons would be a disaster.
But the collapse of North Korea and a subsequent civil war up there, unless it led to one of those named disasters, would not be a disaster for both Koreas.
For North Koreans, the intense violence of collapse and civil war would at least offer the hope that the numbing institutionalized violence of the regime might end.
For South Korea, the price of collapse would be expensive in monetary terms, no doubt. But is it really appropriate to term that a disaster when the alternative could be the continuation of the looming nuclear threat? Would it really be better for the North Koreans to recover and become a more robust threat?
North Korea's collapse would be a lot of things potentially. Uncertain? Sure? Dangerous? You bet. But "disaster" doesn't enter into my play book at this point.
Mind you, everyone can have different objectives. That's the way the world works. But from our point of view, a collapse of North Korea before they get nuclear-armed missiles would be very good, indeed.