I don't think just developing a Z80 operating system is enough. The whole ecosystem needs to be preserved.
In agriculture, we have the doomsday seed vault  just for this purpose. If we anticipate collapse of the current economic system or society, I think we should build a doomsday computer vault, that keeps everything we need to rebuild the computing industry in a underground bunker. It keeps everything we need in a controlled environment, such as 8080, Z80, m68k, motherboards, logic chips, I/O controllers, ROM/RAM, generic electronic parts, soldering irons, oscilloscopes, logic analyzers, schematics, documentation, textbooks, software. We also keep some complete, standalone computer systems such as desktops and laptops, and all the parts that need to service them. We also need to preserve the old semiconductor production lines around the world, although probably not in the same bunker. Even if we fail to build better systems, 8080s are already useful enough!
Meanwhile in peace time, we need to form a team of experts that makes a roadmap to rebootstrap the computing technology for the future using parts from the bunker, with a step-by-step plan, that can be easily followed and executed.
> The whole ecosystem needs to be preserved. Indeed. It takes a civilisation to build an iPhone.
I don't think people appreciate that even within the highest-tech manufacturing industries there is a lot of tacit knowledge. People shake their fists about "technology transfer" to China, and before that Japan; but that's taken decades for them to reach parity. And that's with running, copyable examples and all the parts of an existing supply chain widely available. Similarly the process for making a nuclear bomb can be written in a short paper, but few countries have successfully replicated it.
> It takes a civilisation to build an iPhone. The point here is that we don't need to build an iPhone, we only need a radio. Building a 8080 is much simpler, the USSR did it, the East Germany did it, China did it, all around the same time without too much difficulty. It's certainly would be much more difficult if the current civilization collapsed, but I think the author doesn't anticipate a total collapse, just a breakdown of the current economic system, thus it should be doable..