See that orange slice of the pie chart? When a huge chunk of the productive people are dead the ones who aren't have much more leverage to push back on the usu

See that orange slice of the pie chart? When a huge chunk of the productive people are dead the ones who aren't have much more leverage to push back on the usurious mill fee, market fee, ferry/bridge fee, etc. etc. etc. and suddenly your average peasant (farmer) suddenly has a lot smaller cut of their productivity being taken by the local lords and whatnot. That frees them to farm slightly less and do all sorts of other productive things (raise animals, barter labor with each other, etc, etc) more and increase their standard of living.