If we all cocked our toes up, the country would be in deep trouble. With the population down by 20%, many people who work in the care industry, farmers, shops, clothes and all sorts of other industries would see demand for their products plummet. The housing market would be in chaos, with all those house coming on the market prices would fall - fine for those who want to buy houses but those owning houses with mortgages would almost all fall into negative equity. No new house building would be needed so lots of builders would be out of business.
All that money paid in pensions would be eaten up in supporting those out of work, those industries in collapse and, as over 60 or not many of us pay income tax, VAT, fuel tax and a lot of other taxes, tax revenue would fall as well.