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?
The level of intelligence in the Neolithic was exactly the same as today. The level of science is the difference. And the only way to go back to the Neolithic is to erase all knowledge and all memories.