You have to grow your economy at a rate that allows you to pay for the additional pensioners.
Birthrates seem very hard to change and even if change would happen it is much too slow.
Germany did the experiment on immigration and has not seen GDP growth for half a decade. There the system is failing far worse in some ways, but in Germany employees are paying directly for the pension plans, so the state just raises the employee contributions.
Because Germany does nothing to attract and keep employable immigrants. They only want to keep immigrants which completely tow the line of only speaking German, which inevitably attracts mostly bottom of the barrel. Significant portion of highly qualified workforce in Germany is umemployable because of refusal to conduct business in anything other than German.
Also, the beaurcracy is the real culprit for growth.
Birthrates seem very hard to change and even if change would happen it is much too slow.
Germany did the experiment on immigration and has not seen GDP growth for half a decade. There the system is failing far worse in some ways, but in Germany employees are paying directly for the pension plans, so the state just raises the employee contributions.