So, one reason not to pursue a policy of regime change is that it tends to lead to anarchy and civil war. The other reason is that it encourages maximum intransigence by hostile governments.
Think about it. If you tell a hostile foreign leader, "We want your head on a platter. To settle for anything less would be appeasement," your chances of getting any sort of concession will be none. Any government's primary objective will necessarily be its own survival. No matter what else is or is not on the table, any government the world over will consider this one thing to be non-negotiable. If we make clear to a foreign government that we consider it ceasing to exist is non-negotiable and no concession whatever on its part will be met with a favorable response from us (appeasement and all that), it should hardly come as a surprise that they are unwilling to make any concessions.
Well all right (an opponent once told me), suppose we don't insist to the leader that we want his head on a platter. We are open to his going into exile and will only insist on his head on a platter if he refuses. But that doesn't work for a variety of reasons. For one thing, we are also working on having fewer and fewer countries willing to admit deposed heads of state. For another, if just the guy at the top goes into exile, it is not real regime change, but merely a coup. The name at the top changes, but the "deep state" remains unchanged. On the other hand, if you insist on overturning the deep state, then aside from the dangers of civil war an anarchy, you have countless lower level functionaries who don't have the option of going into exile and will fight to the last because they have no choice.
These considerations are especially strong when dealing with a state like North Korea that has nukes. Saying we want you to give up your nukes, but we still won't agree not to topple you is the sort of proposal unlikely to be accepted. Well that's fine, many hardliners will say. We don't negotiate with evil; we defeat it. We don't want any agreement with North Korea even if they do give up their nukes; we just want their government to be gone. Well first of all, it should be obvious by now that refusing to talk to the North Korean government is not going to topple it. Assuming we can topple the North Korean government without open war, it should at least be obvious that we can't do it on a strict time table. So presumably while we wait for the North Korean government to fall, it will continue developing more nuclear bombs and missiles. Presumably we would prefer for it to spend its last days doing something else.
But even more to the point, why would it spend its last days doing something else if it knows for a positive fact that those are its last days. When the Senior Bush was gearing up for war with Saddam Hussein, he informed Hussein through a back channel that if Saddam used biological or chemical weapon on US troops, we would not stop at driving the Iraqis out of Kuwait, but would go all the way and depose Saddam. Implicit in such a warning is a promise not to depose Saddam if he refrains from using chemical or biological weapons. If our plan is to topple him regardless of what he does, then why not use chemical weapons and to take out a maximum number of us along with him?
Likewise it seems most plausible that the North Korean government, even if it does develop nuclear missiles capable of hitting us, will refrain from using them for fear of retaliation. (Certainly they have been deterred from invading South Korea so far). But if they are absolutely certain that we are going to topple them regardless, then suddenly deterrence stops mattering. Why not at least take as many of us along with them as possible?
And, any number of critics have pointed out, we haven't helped ourselves here. Saddam Hussein didn't develop nukes, and we deposed and hanged him. Qaddafi gave up his nuclear program and agree do to cooperate with us in opposing Al-Qaeda, and we deposed him him and allowed him to be murdered. Now we can explain why that is different and, in particular, that in the case of Qaddafi the revolt had already begun. But somehow I don't thing Kim Jong-un is going to be impressed.
So I will ask again, for the love of Mike, can't we please stop treating some governments as so far outside that pale that no concessions on their part will meet with a favorable response by us? It is a sure-fire formula for ruinous escalation and war.