@emsquared, I'm not sure that I agree. While the "dark ages" aren't nearly as dark as they are popularly characterized as, they were definitely regressive and the period lasted for hundreds of years at some of the most benign estimates. Keep in mind this was also during a period when there were entire castes of people who could read and should have had access to written records. I think it's a mistake to look at human progress as a linear line leading upward as time passes. Each epoch has its own unique challenges as well as its own boons. In the world of SR, where books are obsolete and the near whole of human knowledge, expertise, and technological innovation exists in the matrix, should the matrix fall, collective human history falls with it.
As for small engines, this indeed poses a "problem," if we're wanting to arrive at a post-apocalyptic scenario, but I think you can logically explain a great deal of it, and hand wave the rest. Engines need fuel, and it all comes down to resources. Engines may well exist, but without a steady power supply, they're next to useless, and having engines could very well be mostly in the domain of various warlords struggling for power, a la Mad Max. If the events that led to the post-apocalyptic scenario involved widespread use of EMPs, you could simply hand wave engines away pretty easily, as all of the modern tech would be dependent upon computers to function.
There's also, I don't know, therefore magic to keep in mind.
Last, just because human society would have the opportunity to right its course and bring us back to civilization, there's no reason to believe that one society-crushing cataclysm wouldn't be followed by another 50, 100, 200 years later. Likewise, you can always tweak numbers, so that it's not 9 out of 10 who perished but 99 our of 100, or even 9,999 out of 10,000.