You state succinctly the basic tenet of what John Paul II called "The Culture of Death" - i.e. that people are there to serve some reified abstraction like "the labor force."
The "labor force" and the economy in general is called into existence by and for human beings. It isn't some larger thing that has an existence above and outside that of people that people somehow serve.
As to "who will pay for them" - the answer for conservatives should be "their families." The only place for government in all of that is to help ensure that families can be formed and that they can enjoy safe streets and all of the civil rights (including the rights to self defense, property, and liberty) that their Creator endowed them with.
I would add that your question seems to assume that we have too many people already for some reified "work force." But that's simply not true. This smacks of 1970s "The Population Bomb" nonsense.
Throughout the developed world - Europe, Japan and the North America - the middle classes failed utterly to replace themselves the past 40 years or so. As these childless couples now approach retirement there are far too few youngsters to replace them. That's the real problem we face. There simply aren't enough responsible and educated young people to manage the developed world. It's most emphatically not that there are too many young people.
The failure to have replacement level populations is only a problem in which there is a large social safety net that must be paid for by a work force. Europe could exist for centuries with declining birth rates if it didn’t have a large welfare state hanging around its neck.
Small population growth is the number one sign that you are a modern, advancing state.
Why did people have large families in the past? For two reasons. One children died a lot. Having ten kids ensured that you would have five children reach adulthood. The other is that children were cheap labor. They paid for themselves by working on the farm (or earning money in a coal mine or something).
There were practical reasons for large families that simply do not exist now.