> At no point in human history has living in miserable conditions caused people to stop having children.

That's mostly because for most of history, they didn't have much control over it.

Contraception and basic sexual education changed all that, as evidenced by the sharp decrease in teen pregnancies.

> People aren't generally willing to have kids if it means abandoning their efforts to achieve the most comfortable modern lifestyle that they can.

That's a minority opinion.

Around 12% of the (US) population actually pursues this, in their view, comfortable lifestyle. The rest consistently indicates either same or a larger number of desired children than they have:

https://populationconnection.org/blog/survey-americans-want-...