Demographics is most of it. You need a certain level of productivity to have a certain level of lifestyle. Old people, even if they can be kept busy, will not have the level of productivity as a young person. China is running out of young people, thanks to its a child policies.
There comes a point where your ability to support the welfare state stops, because you stopped having children.
But wouldn’t productivity increases due to roboticization, automation and AI take care of that problem?