When only the rich have money, they keep it. Keynes considered the economy to be demand-limited. If there is a lot of money available to be spent, people go into the market waving their money to pay for goods and services, and suppliers respond by producing more. If there is less money to spend, as in the Great Depression of the 1930's, suppliers respond by producing less. Keynes looked at the depression and said the economy was stagnating because the poor were too poor.
Production requires money to invest in capital and to pay for labor.
The supply side economists said that prosperity was fading away because the rich were not rich enough. I suggest that the economy may be limited sometimes by demand, and sometimes by supply. But you don't have to guess which, because it should be clear from the way prices are going. If the economy is demand limited, then demand will be less than supply, and prices will be stable or going down, as they were in the Great Depression. If the economy is supply limited, then demand will be greater than supply, and prices will be going up, as they were in the late 1970's and early 1980's.