I clarify:
Let’s say scientists can’t come up with solutions to global problems, AI gets out of control and turns almost everyone into paperclips during wars, and in the 2040s or 2050s, the surviving people (about a few tens of millions around the world or even less) gradually return to the level of intelligence of their distant ancestors?
It depends on the region and country, but yes, matalurgy can be resumed over time and quickly in 5-20 years, again depending on the country and region.