They may very well find that doesn't happen. Social contracts work both ways.
If you don't want children, make sure you are the best aunt/uncle in the world.
Many have retirement savings, and so on, that are merely investments on time they've worked. That work has also created the opportunities for the next generation to occupy.
The idea that there is some obligation to reproduce is a little antique, and very much out of keeping with the environmental impact (an environmental social contract) of increasing population sizes.
You can’t eat “investment”. When you finally start using these retirement savings, it will be to buy goods and services that will ultimately be rendered to you by someone else’s children. You’d better hope that by the time your generation is retired, next generation is large and willing enough to support you to the level you expect. Savings are just claim on a share of future production, and if future production is to be reduced in aggregate (and with population falling quicker than economy is growing, it very well might be), your claim will be worth less than you expect.
In the end, our generation will be cared for in retirement by our children and grandchildren either way, and everything else is just accounting.
You are making a speculation that your children will become a high-functioning adults that will be capable to produce more than they consume.
Overpopulation (as a real danger to our planet) aside, people who are opting out from having children are statistically highly educated and more successful (produce order of magnitude more than they consume) meaning that goal and career oriented individuals are feeding your children at this very moment altruistically (without any guarantees of payback from your children when they reach adulthood).
That's crazy talk.
The further we, as a society, distance ourselves from basic, primitive instincts (like reproduction) the more advanced we become (which is not simple or easy at all, it requires significant cognitive effort (goals, deliberate education, atheism)
Most of the pleasures that you enjoy in modernity were delivered to you by high-functioning, educated adults without children, while your children have a very high chance of becoming consuming individuals who habitually take advantage of the generosity of others without making any useful return.
Well, that's an assumption you make. Me and my wife don't want children but we are also working to build sustainable future for ourselves so we are not much dependent on future generations. Pension funds, sustainable business for the future, low footprint. And yes, we are paying into the system which is ripping us off today anyway. So you, know, we finance your parents, your children can pay something back.
This obviously depends on the country you live in. In Germany, there are talks to increase pension contributions for people who don't have children.