I think the problem is one of bootstrapping. With PLCs controlling even sewage pumps society will have a lot of more important things to do before we come around to fixing elevators, automatic doors, or even chip fabs. Then there are herds of forgotten VAXen and AS400s that have been chugging along for decades running who knows what mission critical code, all gone without as much as specs remaining.

The less basic infrastructure which is wiped out the faster we can rebuild. Right now it looks like we will be falling back to whatever cam-actuated automatic lathes which have not been sold below scrap value that are left. And I hear those things are a bitch to set up.