Young generation is guaranteed nothing for their contribution.
It could only be sound if population is to grow organically and indefinitely, which will obviously be unsustainable either.
Modern economy works in this way: you give up ~40% of your income, which is then redistributed between retirees. Infrastructure and all other kinds of social programs also take their cut. This works when there's two workers for every retired person
If there would be as many retirees as workers, you will end up giving 80% of your income. But it is totally unsustainable. Imagine that you pay $10 for a coffee, and the barista only gets $2 towards his account, post-tax. She won't be able to make decent living. You strangle life out of economy and make it go to grey zone.
Yes we may be able to produce endless amount of wealth, but we are not able to make this sheet balance itself. There's just 100 of percents and they don't make those anymore.