I suppose you could say many things have failed due to no one seeing the big picture among all the tiny details but without people focusing on the details none of that would have been possible to begin with.
I'm not a historian but I can imagine that a medieval specialist, such as for example blacksmith, could very much fix his own house (or maybe even build a new one), grow food, take care of animals, make wooden tools, make some simple herbal medicine etc. I'm guessing a lot of he was consuming was produced by himself and his family. Today, for engineers it's the total opposite - we only consume things produced by others, and essentially by strangers whom we've never met.
Regarding fragility specifically, if a major crisis comes (say a war), the desk jockeys, esp. ones didn't have the wisdom to accumulate resources, will fare much worse than someone who's say running a homestead. If they did accumulate resources (provided they don't become worthless by hiperinflation etc.), they will be able to buy the essentials and hope their stash lasts until the end of the crisis.
You should read Collapse Of Complex Societies by Professor Joseph Tainter.
Tl;dr a step change in complexity of ancient societies more often than not lead to their downfall. They were unable to train/retrain specialists quickly enough to deal with surprises.