> any genetic trait that causes people today to think "You know, I think I'm going to remain childfree" is specifically selected against

More specifically, the genetic trait of being susceptible to the childfree brainwashing. People aren't born not wanting to have kids, since all their ancestors did.

One reason our ancestors had plenty of children is that children were a large net-benefit to them.

Many modern societies are structured in a way that makes children an economic negative to their parents. From an individuals' POV, the "smart" play is not having children and hope others do.

Individuals believe that they do not need children because they have saved up for retirement, not accounting for the fact that if everyone did this and there was nobody left to do any work once they reach retirement age, that money will be worthless and they'll be unable to retire. The alternative is mass-scale immigration, and that's becoming unpopular. For an example of what happens if a society has neither enough children, nor immigration, see South Korea. Poverty among the elderly is rampant and they are increasingly being forced back into the labor market.

Many western societies needed to start applying a large multiplier to parental benefits decades ago.

>People aren't born not wanting to have kids, since all their ancestors did.

What evidence do you have that all their ancestors wanted to have kids? I can think of a lot of reasons why people may have had kids without necessarily wanting them. They wanted to have sex in an era before birth control. They were pressured into it by the social norms of their local community. They were raped. Etc.

I have a lot of criticism of the childfree movement but I think it’s pretty disrespectful to assume they’re not being honest when they say they don’t want to have kids and that they’ve always felt that way.