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?
There’s plenty in landfills to be “mined”
Afaik that can only be recycled with pretty energy intensive arc furnaces or regular fossil fuel combustion. No accessible fossil fuel limits you to fallout style scrap-tech, which is probably not sustainable for a long term 17-18th century civilization.