I don't really follow? Workers are supply (their labor) and demand (their consumption), but as we have more productive industrial capacity and more of it, the cycle breaks down as not enough labor is needed, reducing their purchasing power and further slackening the labor market in a vicious cycle.

Retirees don't work, which means nevermind the absolute amount of demand (no kids, etc., make less per household sure) the ratio of supply to demand offsets the productivity / industrial capacity gains. Coupled with more retires, that could restore the balance and get the "Keynsian feedback loop" going again.

(We could still go into a massive worldwide recession, but that would be a political result of the powers that be not wanting the profit share of national income to decrease. That is plenty possible but not some unavoidable "demographic destiny".)