It’s temporary. The humans of the future will be the descendants of the people having lots of kids right now. You should basically never extrapolate out any trends that are driven by people with below replacement fertility rates.
Let’s consider fertility using this lens:
1) Fertility rate is heritable, 2) Fertility rates vary from person to person, and 3) People with higher fertility have higher reproductive success (it’s almost a tautology).
I’ll let you work out the implications.
No, it doesn’t. The environment we inhabit has changed drastically in the last 150 years. We were well adapted to the environment we lived in 150 years ago. We are poorly adapted to the environment which we live in now. That will change as evolution does its thing.
For a glance into the future, take a look at the habits and lifestyles of people with lots of children. There is an interesting bi-mods distribution, btw.
Their adaptations will spread. And, thus, we shall evolve.