Unless they are getting immigrants from off world, that is at best a game of musical chairs. Birth rates are plummeting globally.
Sure, you can still find nice big numbers in places like Niger in sub-saharan Africa, but even they are starting a sharp decline. And they are starting with a low base. Their entire national population is only the size of the greater NYC area.
People often say you shouldn't have kids for your own retirement. In the ultimately hypocrisy, they often mean kids will be for everybody's retirement (when they're forced to pay social security taxes as adults).
The economic incentive is to let others have kids and shoulder disproportionately the costs, while you can free ride and "not have negative consequences." Immigration doesn't solve this problem at the system level as removing children from foreign tax pool into our domestic pool shifts benefits from one elderly person to another.
The question is then who will care for these people when they get old? Other people's children? So other people should have children, put the effort to raise and educate them for your benefit? This is a very egotistic view.
At a societal level, is an imbalanced population pyramid a problem? Yes, and many countries are solving this by opening up immigration options.
Should an individual opting not to have children be expected to have negative consequences in old age? IMHO, no.