I remember when many alt coins collapsed in 2013/2014 one problem was that with miners dropping off, difficulty stayed. Many coins with longer difficulty adjustments couldn't get back to reasonable confirmation times.

So let's say BTC difficulty drops 10x (China cracks downs on farms or simply miners moving to a different more profitable coin, who knows)

Currently this would be ruinous, as difficulty adjustments are every 2 weeks, so that would be 140 days until things get back to normal.