As for the burden of having kids, I think it’s way too simplistic to say it was a burden NOT to have them in the past. There were different cultural dynamics, for sure, but kids have always been a mixed blessing—both difficult and extremely rewarding. Having kids has historically forced adults to face their own shortcomings. Not an easy thing to do in any era.
...why do you assume it would be yes?
But if, as hotpotamus suggests, "consent" is a useful way to look at this, then that's not good enough. You can't randomly do surgery on people just because you believe there's a 90% chance they'll be happy with the results.
Someone is unconscious and dying, you can save them with surgery, surely it’s highly unethical to wait for their explicit consent?!
If the ethical thing to do is increase the total happiness (or perhaps contentedness, or satisfaction) of all humans...well it's just not really clear that adding a statistically average human is the way to increase human prosperity / reduce human suffering.
To your other point, now that I'm a parent, it's easy to see that becoming a parent made me into a better person, but there was no way I could have known that that would happen before becoming one.
All of history has been an steady march forward of more prosperity and less suffering accomplished by an entire population of, on average, average people.
We're really really far from extinction. That's just simply not an issue at all.
Something something boson something neutron, and the Earth goes kaboom.
Children can be very dangerous.
Why do rich people in rich countries have fewer children than poor people in rich countries?
Nowhere is it seen that more money correlates to more children.
I don't think it's really a question of "enjoy your riches," because no matter what your status, nobody can "afford" children, not even the ultra high net worth individuals. It's a burden regardless.
But I think therein lies the answer. There was a time not all that long ago that not having children was a burden. Who was going to bring in the harvest, or at the very least supervise bringing in the harvest? Who could you really trust to take care of you properly when you got old? And of course, there's a biological compulsion as well, but that's much easier to sidestep only in the last couple generations.