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?
In the novel Star’s Reach, in a post-collapse society, one of the main characters is a ‘miner.’ Their mining guild basically tears apart old concrete buildings, by hand, with sledge hammers to extract the old rebar from them.