Raising your child will run you ~300,000 USD, in addition to the massive opportunity cost from time investment. Given that social security is funded entirely through taxes in that tax year, it's a bit much to expect the children of 53% of the population to care for 100% of the elderly when the willfully childless 47% of the population did not put in more upfront in some other way. As birth rates drop, the ratio of workers to retirees will skew worse and worse, and the entire safety net will collapse progressively.
Alternatively, we can encourage and incentivize child rearing. Yes, that means that the willfully childfree will need to pony up resources for those who want children. What would you prefer: pay now for the continuity of the safety net, or the collapse of the safety net?