Evacuation of Seoul would create an undeniable signal to P'yong Y'ang that the balloon is about to go up. IF Kim Jung Un is as ruthless as he is routinely portrayed in the Western MSM, then he will not hesitate to attack. Better, in his estimation to go down fighting, causing as many casualties as possible than to submit with a whimper. However, that is a might big IF.
Five years is still five years. It allows a more measured response than the panic that el Presidente has created and largely stoked. Action, if necessary could be two to four years down the track. Negotiation is much more likely and much more fruitful. As distasteful as the idea of talking to Kim Jung Un is to some people, it appears to be the only way forward to actually achieve something.
The DPRK has, as Steve Bannon suggested, realistically, "there’s no military solution here, they got us.” Until some people recognise that the use of military force is limited in what it can actually achieve, and it's inability to actual prevent the DPRK from destroying a large number of people and a great deal of property. Does that mean we have to answer their ransom demands? No, not completely. Does that mean we are going to guarantee the existence of the Kim regime? No, it does not. What is required is a co-ordinated, comprehensive and extensive response.
What is clear though, is that a coalition is required and petty arguments will only prevent that from being created effectively.