Not for a looooong time to come. It’s going to take a minimum of 20 years, if not more, for the ROK to bring the DPRK zone up to the same level as the South in terms of infrastructure, housing, health care, and industry. Look at how long it took a reunited Germany to get the East up to speed. It’s going to be ten times worse for Korea — not even taking into consideration that a good percentage of the population is full gonzo NUTS.
Someday, they might cause Japan to worry. But it won’t be in my lifetime, that’s for sure.
The article also mentions the savings to their economies by not having to support two huge standing armies. That there will put a lot more people in the work force.
Politically it will be difficult trying to mainstream a large brainwashed population such as NK has into the 21st century and that will cause a lot of strife and will probably be the most difficult part of reunification. Unlike the East Germans, who had a very good idea of what things were like on the other side, the NK populations is still largely brainwashed.
The article also mentions that Russia and China are already working on building the infrastructure up in NK as they are both anticipating a major change in NK. Between the SK, China and Russia I don't think it will take as long as you think to to get NK up to par. Russia has a huge source of natural resources that is readily available to NK and they have improved the train tracks going into NK and the Chinese and Russia are already modernizing ports.
A lot will depend on the condition of the NK people. If they are educated and smart, things will go quickly. If they are illiterate and mentally stunted from malnutrition, then NK reunification will NOT go well.