If you are retired then in order to cover expenses every month you either a) sell assets b) receive payments from assets. If you are invested in an index fund

If you are retired then in order to cover expenses every month you either a) sell assets b) receive payments from assets. If you are invested in an index fund and the market nose dives for 20 months you don't have the luxury to wait until it recovers. You are forced to sell at 30-40-50% lower values while the slump lasts. This translates to burning through your nest egg at 1XX% the rate you 'normally' would for the duration of the slump.