The number of feet to wear the shoes is irrelevant if they have no money to buy them. Business doesn't need more people to prosper, business needs people to prosper more. Although most systems of providing for retired citizens are financed by taxing working citizens, the concept of needing younger people to support older people is obsolete. If used responsibly, products from the industrial and technological revolutions could satisfy our needs without selling our children into wage slavery. Social security systems are artificial, so adjustments for changes, such as a reduction in the number of potential workers, can be made.
Automation removes more workers from payrolls than birth control does.
Adjustments could be made. Unemployment reveals that we already have enough potential workers. Increasing employment and increasing wages will increase funds paid to social security. More money is taken in for social security than is shelled out, but the remainder vanishes instead of being invested for future pensioners. The solution to having our nest eggs stolen isn't to lay more eggs.