Demand and supply side are two ends of an economic cycle. Cycles start out as demand economies as wealth increases for the middle class. As time goes on the wealthy constantly trickle up wealth until the only demand is caused by wealthy spending, such as housing and office buildings and middle class consumption is mainly through loans. When the consumers are "loaned out" the end is near.
Capitalism depends on the middle class consumption enabling entrepreneurs to start businesses that have a demand. Adam Smith warns about starting something when no demand is present or possible.
Preserving a demand side economy requires attention and work preventing concentrations of wealth taking consumption out of the consumers' hands.