exactly, you can have all the paper money in the world, if there's no actual human beings to do the work, the money is useless.

declining population doesn't mean there will be no young people at all, just slow decline with less and less people including less old people not replaced by anyone

pensions don't need young population at all, you can save your own money for retirement without anyone's contribution towards your pension

>you can save your own money for retirement without anyone's contribution towards your pension

No, that won't work. Money is only worth as much as the goods and services it will pay for, and those goods and services have to be provided by the labour of the working young. If there aren't enough young people to meet that demand for labour, prices will rise, and the purchasing power of those pensions will evaporate.

(This is why the "triple lock" and other such schemes are so evil; by pegging state pensions to inflation, they unfairly lock in the purchasing power of a lucky generation, in defiance of all economic logic.)

I think the terminology you are using might be confusing to some. If I am correct based on your adjacent comments, what you mean by “pension” is the societal agreement to provide for older/disabled people from taxes.

If so, then yes, getting rid of that societal agreement would solve the problem of decreasing productivity.

yes, only you should be contributing towards your own pension without state (financial) assistance, it should be your savings/investment pension account

your kids can help you voluntarily without any system involved

if you didn't work/save and don't have kids them you can get some basic support same as people in productive age get, but it's not really enough even to survive