I should clarify. There are two separate issues here; Providing the necessary resources and support for people to have a dignified, comfortable old age is good and important. We absolutely should do that and it is an important priority.

Separate from that is the idea that we should create more people to care for existing people. This is generally phrased as though existing people are owed the labor of people yet unborn, and that creating more people specifically to benefit from their labor is morally neutral or good.

A dwindling population will create hardships for people currently living, but I do not think that justifies or necessitates creating more people specifically to maintain the status quo. Having kids for the joy of having children in the world is good, but there's something very perverse about John Capitalist telling you to have kids specifically so that John can benefit from their labor right up until he dies and doesn't care about any of it anymore. There's an unspoken "I got mine fuck you" where John Capitalist is only worried about lower populations in so far as it will effect him and the systems he thinks are important.