I suppose I basically agree with you, if the only way forward for NK was welfare...
Another perspective, though, is that there are companies that are DYING to invest money into NK because what it actually represents is massive, latent economic output which is currently handicapped because of its government...never mind the hundreds of thousands of SKs that have relatives in NK they'd like to reunite with. While it would take "trillions" to repair it, I think the ROI would be positive. Take a look at SK for two seconds...what makes them so different? Nothing. 0.
Currently, we're "investing" much more in Africa, which has two things: far more potential, and far, far more cost to getting up to the modern world...yet no one scoffs.
NK is NOT far behind what East Germany was post war - they are a nation that currently has the ability to produce nuclear weapons and ballistic missiles, yet most of the population lives off of rotten cabbage and a little tiny rice bowl three times a week. They have every bit of potential to be as productive as SK given the opportunity. A lot of trade and "economics" already happens between the two states. Their problem is 100% political.
The problem isn't how much it would "cost" - the problem is letting a festering wound continue to fester. Worrying about a 1.2 trillion dollar "hole" misidentifies the root cause. Here's an article calling Corona a $16 trillion dollar event (https://edhub.ama-assn.org/jn-learning/video-player/18553761#:~:text=The costs of the coronavirus,costs%3A the %2416 trillion virus.). Do you really think Corona is going to end the world...really?.. REALLY? Get some perspective. 20% of all money has been "created" in the last year (https://moguldom.com/310861/strategist-almost-20-percent-of-all-u-s-dollars-were-created-in-2020-alone/). Worrying about cost is cute, but that position has to ignore the broader context.
What is the cost of continued conflict in that region for the next 350 years? Hint: far more than 1.2 trillion.