The essential problem is that the people working now are paying for the people that are retired. It would make more sense for the gov’t to have taxed the people prior to their retirement, and have invested those taxes, so that in their retirement they would be getting out what they had previously paid in. And switching over to a system like that would require double taxation on the population now, which will make such a proposal very unopopular.

But if your retired population is growing, and you have fewer people working, then you either need to increase the retirement age–so that more people are paying into the system–or increase the taxation overall. If I recall correctly, Denmark has been seeing a negative population growth; that’s a real problem for retirement schemes that rely on current taxes paying for retirees.

Is this fair to people that have been working in trades and have beaten up their body for 40 years? No. Likewise, it’s not really fair to people that have working in white-collar jobs that may still be more than capable of excelling at their job, and still want to work. (My dad had mandatory retirement at 72 due to company policy; he immediately got re-hired as an on-site consultant, and has been doing that for over a decade.)

EDIT - this is a huge problem in the US. The social security taxes now on working people are immediately paid out to retirees. SS benefits go up to account for inflation, but the amount coming in is decreasing because population growth has slowed. Without major reforms, social security in the US won’t be solvent by the time I retire, IF I ever retire.