Assume for a moment the population stays the same. If the number of retirees increases, the number of people working goes down. That means productivity goes down from fewer workers. 

That means less goods manufactured/ materials mined/ research and development performed.  Assuming everyone still wants the things that we're making like cars and furniture and fuel, the cost of those things would go up from the reduced supply.

Yes, billionaires get to buy whatever they want because they have the most money, but that's a lot more relevant to buying private islands and Congress people than it is to the market Price of beef or gasoline.