Grandpa probably could have been entitled to his own generation's money[...]
Actually no. Money is only worth what you can buy for. The work, the good or the service Grandpa wants to pay for has to be done right now for today's prices. And while people working today also get today's payment, Grandpa has no negotation lever on today's pricing. He earned his money in former times at former prices, and now he is retired. If the older generation which doesn't work anymore has too much money, we the working generation will (free market to the win!) just increase prices until the purchasing power of the older generation fits again the amount of work we want to spend on them. If there is too much money on the market, we always can have an inflation until purchasing power and goods creation are in balance again. Working people can deal with it thanks to increasing wages in an inflation. Retired people can't. Their retirement funds compete against the retirement funds of all the other retired people, but the share of goods and services they compete on is defined by the people still working.
Interests, payouts for 401k, house prices and all those money sources non-working people may have access to are only possible because people today are creating the surplus value which can be paid out as interests, as profits on shares or be spend on ever increasing house prices. Every retirement scheme where one stops working and still has access to goods and services is in a way a Ponzi scheme because someone else is creating the actual value the retired one is using up.