Plausibility aside, while the financial costs would be immense, a reunification could have the potential to alleviate some of the most pressing issues for both North and South.
For the North, they'd not only have access to international markets for the first time in decades, they'd actually have access to food and medicine, proper forestry, and a boom in industry. Southern Korea has the majority of the peninsula's arable land, which had disastrous consequences for the North since the end of the Korean War. Reunification would see the North actually have access to food and medicine, which would see their population actually grow. For forestry, lack of technology and infrastructure force many North Koreans to rely on wood burning for heat during winter. As a result, much of North Korea has been deforested, which makes landslides during storms more severe, reduces natural wildlife (which suffered from overhunting during famines), and reduces construction supplies, among other things. Reunification with the South and access to its electrical grid removes that as a factor. And for industry, the North has a large population that doesn't have higher education, is used to long hours, and has historically had low wages, plus it has something like $1 trillion in ores. Those combine to make it a potential industrial power. It is, however, hampered by lack of technology/expertise, infrastructure, and capital. All things the South has or can provide readily.
For the South, it would have access to a large, cheap labour force that already speaks their language, bountiful natural resources, and the potential to stop focusing everything on Seoul. While cultural integration would take time, having 25 million additional workers that speak Korean and have low wages would remove the need to outsource to other regions and speed up production. There's also many more potential farmers who can replenish the South's diminishing rural population. Natural resources have already been mentioned. And Pyongyang historically was a rival to Seoul in importance, being the capital of multiple Korean kingdoms and the biggest city in the North (rivaling Seoul in terms of population at times). It is also close to China, which was part of the reason Incheon exploded in importance after China eclipsed Japan as a trading partner for Korea, which in term saw Seoul completely dwarf Busan. And this ends up being beneficial since that means Seoul isn't the sole core of South Korea and housing prices don't go quite as insane. And that has knock-on effects for population growth, since lack of economic opportunity and high cost of living + housing are major reasons for South Korea's population decline (there's another issues, like lack of maternity rights and support, but that's unlikely to change in this scenario).
Of course, it's horrifically expensive (and the longer it doesn't' happen, the more expensive it becomes) and undoing generations of a cult of personality would take a generation or two to do in general. Plus, South Korea has one of the highest suicide rates among developed nations and North Koreans in South Korea tend to have a hard time integrating (hence redefectings, suicides, etc.). But after time and investment, reunification would've (and still would) see Korea explode economically and demographically and easily end up as one of the G8 (perhaps overtaking Japan as per https://www.wsj.com/articles/SB1253...hs_Has_a_Different_View_of_Korean_Unification
The status quo for both countries is less than ideal, besides. North Korea is languishing under isolationism, with all the cultural, economic, and demographic consequences that entails, and South Korea is burning its population out, with estimates seeing its population decrease by 50% within 100 years just from lack of population growth. If current trends continue, the peninsula just ends up as a backwater again, its brief time in the spotlight extinguished for the foreseeable future.