[Subjectivist]

Did the hunter gatherers and peasant farmers of Yucatan remember how to build Mayan pyramids? Did the farmers and shepherds of Italy in 1000 remember how to make the plumbing in Rome work and maintain all the civic structures?

How many literate people were there among the Incas? And how many books or libraries? Was Rome ever totally abandoned? Sacked? yes, but reduced to non functional rubble?

We are vearing pretty far off topic here,

All of our best discussions do.!

my only point was you need more than basic knowledge to maintain a complex civilization. You need resources and the will to employ them as well as the knowledge of what to do.

But we will not be without resources. They maybe in much shorter supply and we will have to find ways to make do with less but I do think large groups of people do have the knowledge and will to do that.
That is not to say that we are not on the verge of a population crash which will change our world dramatically and at the same time reduce the demand for resources but it will be nothing like returning to the thirteenth century.

Sure some places will have enough resources, and some places will retain knowledge for how to use them, but unless someone in charge has the will to add A and B and force people to put them together the people will slide back even further and the knowledge could be lost.