The evidence that North Korea cannot sustain itself is the sheer amount of foreign aid it used to receive (and still does). Even if there was a reunification, the education and culture of North Korea leaves them largely unable to cope with life outside according to people who have interviewed North Korean refugees.
Fully 10% of the population died during a food shortage in the 90s. in fact, since the country was split, North Korea has never sustained itself without most of its food imported from other countries starting with Russia in the 80s. When the Soviet Union collapsed and stopped sending aid, China stepped in and provided more than 70% of both food and fuel to N. Korea up until the floods when the US and UN also provided millions in aid to this day. Their current situation is mostly Russia's fault historically by lack of administration, but now all of our countries have a part of the blame for continuing to send aid. If they have money for nukes they can make their own food.