Economically speaking, it would add millions of people to the working stock of south korea, plus extensive land to build new cities and such. It would guarantee decades of profitable investment and growth.
Long-term I'm sure it would be profitable, but would South Korea dare make that investment in their current period of tremendous growth? The opportunity cost loss is massive. Would US/China help invest?
In principle, I would not make the government pay for anything significant and it would sort itself out.
[0] https://tradingeconomics.com/south-korea/youth-unemployment-...