It's not actually so clear cut. In all times there was a huge sways of people who won't have kids.
Handmaids, sailors, mercenaries, servants, slaves, etc, etc. Some of them could have children but it was never guaranteed.
This was offset by other people who'll have more children. Sometimes it wasn't. At all times a lot of lines will wither. Some vast tribes will be reduced to a few dozen families.
Social contract is XX century construct and obviously unsustainable at that. Once children stopped being source of labor but labor sinks, it tried to also became unconscionable contract. As in, everybody tries to slack off their duty while praising it to other people. It's like with conscription.