Understand that we are talking here about a scenario where the global fertility rate drops significantly (less than one child per woman), like what is happening now in South Korea, Spain, Italy, ok?

Given that scenario and the economic collapse that would be coming with it, what would you be spending on, exactly?

Has it occurred to you that all that money that you have been saving and putting in some pension fund will likely not be there if there are no younger people to keep the economy going?

Also: what is stopping you from "living your best life" now that you are young?

Lastly

Use the hundreds of thousands I would have otherwise spent on kids

Yeah, that is not a thing in Germany. Decent public schools, decent health care system, government gives you 250 euro per kid per month, no car dependency, which means that kids are a lot more independent a lot earlier in life... having kids does not cost that much.