I thought we were talking about the US specifically and not "some countries". Whatever issues Italy and Germany might have, the situation in the US is different, and all I've seen suggests that Social Security is financially secure for the next decades at least.
And regardless, declining fertility rates are not a fatality, but also something that depends on policies. If you want to get serious at dealing with that, a big part of the solution is providing women with the resources they need to reconcile motherhood and a professional life (eg public childcare, long maternity leaves, etc.).
None of those things are going to increase the birth rate.