The max FICA (Social Security) tax in 2017 was $7,886.40, which equates to an income of $127,200 (it's a 6.2% tax). There are many people making below that threshold, and not even paying that much.

You can't live very comfortably on less than $8,000 / year. It takes nearly two workers (maybe three if you consider the average annual income, and that the fair majority workers aren't making maximum FICA payments) to cover every retiree -- which seems to logically require successively larger working age adults than prior generations.