IMHO the regime won't collapse unless you see US & ROK troops north of the DMZ and moving north. The regime survived a famine where people ate bark off the trees and grandparents let themselves starve to death to provide rations for their grandchildren not long after this time. There was (and is) no question that the USA and the ROK would be unable to eliminate the DPRK and reunite the peninsula. The questions would be the cost, both to the US and ROK military but also the civilian population in the ROK (particularly the Seoul metro area) and the damage to South Korea industry and infrastructure (50% roughly of ROK industry is in the Seoul metro area). Internationally the response of China was an issue. More "occupying for humanitarian reasons, temporarily" a chunk of the DPRK to prevent US and ROK troops along the border rather than a repeat of the Korean War with Chinese intervention, but still a possibility of inadvertent clashes. DPRK use of chem or bio agents is certainly possible with all sorts of consequences.
While the US and ROK are allies, it is always worth asking if the ROK would go for this sort of preemptive strike and potential major conflict where, to be honest their troops, their civilians, and their industry/infrastructure would take the overwhelming bulk of the hit. The ROK will be asking oif the USA is going to pony up cash to rebuild ROK damage and/or money to cover the costs of reunifying (which would be much more than the absorption of the DDR into the FRG).