We are not nearly in such a resource-counting phase: We produce more than enough food to feed every single person in the US several times over: we have enough empty homes to ensure every person is housed; we have enough of much everything. Thanks to innovations that have happened under Capitalism, we are less resource-constrained in this regard than ever in human history. We have great excesses that are wasted, sometimes to our detriment, and sometimes comedically (ex., the practice of burning tonnes of corn by farmers in attempt to keep prices higher).
That Social Security is unsustainable is not due to a lack of physical resources nor manpower, but capital. Money.
There is currently a cap on taxable income, for Social Security. For the first $168 000, you pay the Social Security tax; for income beyond that point, you do not.
If we lifted or even abolished the cap, Social Security would be more than above water, and no cuts whatsoever would be necessary.