NK defensively also has a significant force multiplier in the form of the terrain - if it was like Iraq they wouldn't even last 5 weeks (on the offensive probably not even 2 weeks) - but they are dug in and could last months defensively against anything the US could throw at them short of heavy use of nuclear weapons - even their bunkers, etc. aren't easily compromised as some were claiming as they are dug in deep (often former salt mines, etc.) with multiple entrances connected by long tunnels.
The terrain indeed in their advantage but I still don't see it as much of a problem as many see it, It's likely will become a major problem for NK forces as it be just as much of a disadvantage to them as to any Allied forces since they won't control the air, so anything moving on top will be bombed, that means no supplies for any forces hidden or trying to wage war while Allied forces will be free to be transported anywhere.
Bunkers and Salt mines isn't really a problem too, entrances will be destroyed trapping them inside or you just drop MOAB's on top of them which will destroy the mine whatever the dept due to the shockwave alone.
They've spent more than 30 years working on those problems amongst other things their army is designed to be able to fight divided up into semi-autonomous divisions with supplies fairly localised as well as a network of sunken railroads, etc. a lot of the country looks something like:
With densely packed valleys - the bunkers are often under 100s of feet of rock with multiple entrances in adjacent valleys - sure they aren't impenetrable but they'll be tough to crack as well - there is a variant of a MOAB designed specifically for targetting hardened bunkers at depth but even that isn't a guaranteed kill against these.
The elites of NK ain't stupid, they understand the outside world unlike the poor sods who are not allowed to leave.
This is actually the most dangerous aspect of NK - if they ever managed to develop an end game nuclear arsenal that they could use with relatively impunity they will use it - unlike most other countries that don't have the siege mentality at the level higher ranking NK people do.
My example was a theoretical end game scenario where they could first strike wipe the US with the press of a button and take maybe the odd nuke in return. I think its the main danger of NK that their mindset is much more towards that end - while even say Russia if someone gave them the button to wipe the US off the face of the earth tomorrow with relatively little consequences would be highly unlikely to use it (sure there would be a fair bit of geopolitical realignment to the new reality).
People who speak about how big North Koreans army is should refer to Desert Storm for how little that matters these days. The Iraqi army at the start of Desert Storm had at least 4,500 tanks, 484 aircraft, 232 helicopters and 650,000 regular troops. They were defeated in 5 weeks losing the majority of their military hardware
Flat desert, is literally the easiest terrain on the planet. (well mostly, dust isnt fun)
or maybe they just turn on their glorious leader when china whispers promises into peoples ears...china can promise to modernise the whole state, infrastructure projects for chinese companies with NK people doing the work, revitalsie the economy and bring them into the modern age
its only the leadership holding them back...it would benefit china to make it a true vassal state, opening up trade routes and still giving them a buffer between their border and south korea
they might have the excuse they need and nobody would stop them
Pyongyang is largely an exception - a lot of the rest of the country is like my screenshot above - and even then around Pyongyang is not ideal terrain for anything other than attacks from the air - lots of bottlenecks in open terrain (i.e. due to the farmland meaning dikes around soft ground being the only path) where an invading ground force would be pushed into bottlenecks where they could be engaged from dug in positions.
Sure they can't win but the mentality they've been brought up into means that while there is a half effective backbone of those totally brainwashed in command they will fight on for quite awhile.