No country that I'm aware of (except maybe Norway and the Saudi's, because they have resource wealth?) fully funds future social welfare policies from present day revenue.

Consequently, future generations will always be paying for present day late life social support.

Because of this, negative population growth requires higher and higher present day tax burdens to continue paying equivalent benefits.

And population economics are not something you can paper over, because they're by definition on the order of your entire country's economic output.