2. Technology lost from major conflagrations is not easily recovered, and may not be recoverable at all. Petroleum obtained using the tools of the early 20th century has been used up. Petroleum that is available today is beyond the reach of the tools used 100 years ago. To recreate such tools today probably requires access to resources that are available only using today’s tools. IOW, wars and other conflicts may indeed push civilization back to life as it was 150 years ago. If they succeed, the path to recovering to 21st century life may be necessarily foreclosed.

If the conflagration occurs after the appearance of the universal assembler, then it won’t be a problem.