At one point we had a long back and forth with my cousin, a post-apo fan, about the credibility of various scenarios, various shortage, various technological regressions. My conclusion: if humanity loses the ability and the knowledge to make CPUs, then CPUs are not the first thing you will miss.

It would have meant that a generation-long obscurantist crusade would have purposefully destroyed that knowledge.

I don't see anything natural nor a human-made disaster that could durably erase all knowledge and industries on a global scale. You would need an intelligence geared at destroying knowledge specifically.