The main reason I said that is because it's an extremely difficult time to be a kid and a parent. For many reasons.
The word I would use is probably "obligation" instead of "purpose". And I don't mean that in a bad way, I just mean that it ultimately doesn't matter if I get any sort of personal fulfillment out of child rearing or not, but I damn well better do my job and take care of that kid to the best of my ability.
I know a number of childfree individuals and couples...some by choice, some not. They are doing some pretty sweet stuff. One couple retired at age 40 (FIRE for the win!) and is selling their place and hiking the PCT next summer. Another does a ton of volunteer work for nonprofits that I would never have time to do. A third is a closer companion and caretaker to my daughter than anyone else besides her myself and my wife.
Anyway, in an attempt to keep this on topic, my main point is that nobody should have kids because of some societal obligation to train our replacements and caretakers for when we are old. If the government thinks we need more young people then there are millions of them who would be more than happy to immigrate.