People simultaneously want to have fewer children, to not have many immigrants, and to have high pensions for long time.

It is obvious that if there are not enough young people there won't be enough money to pay pensions for the old, the old people who do not want to work until they die should live with their children, and take care for their grandchildren, like people did for millenia and still do in many countries where government interventions didn't break families yet.

An additional benefit is that spending time with grandchildren increases the lifespan, and is most likely the main reason why people evolved to stay alive after passing childbearing age.