No children: no society. Or: a very old, geriatric, undynamic, fearful society with no innovation. Simple.
What happened that after a few hundred thousand years of having families we suddendly need to be "absolutely certain" to have kids? Human nature is the EXACT other way round. What we currently see is an outgrow of pure hedonism. It kills societies (literally, at least in the sense of "no new life").
Concerning "being fully aware that our previous lifestyle would be no more": this in itself partly shows the problem. So our casual life choices are in such a way that it is completely uncompatible with having a family? Sure - when you're 20, you might not be ready. But do we, as a society, promote lifestyles that even when you're 30 are impossible to combine with having kids? Then maybe our lifestyle as a society is wrong, what we tell ourselves to be "good life" is wrong.
And then again... maybe we take having kids too seriously? I really can't say having missed out on much since having kids (and no, we have no nanny). We simply did the stuff people "fear". We went out for dinner together two weeks after our first one was born (together with the baby). And we repeated, until today. We went on those 10 hour flights. We explored foreign countries. We visit museums with modern art. We spent months abroad. Is it always easy? Nope. But it was, overall, not too different from the life before. Kids get used to stuff, parents get used to stuff. Most of us can do it if we let go of the instagrammy thought of having to be perfect. And this is how society always was and always will be.
EDIT:
In the current medical climate where child mortality is very, very low, of course, as a society we CAN afford to have less kids overall. And as long there are a few people who LIKE to have 3 or more kids, we can afford to have some more people having no kids as all. I don't promote growing society ad infinitum. But having sub-replacement birth rates will NOT make our countries better for sure (and the economy is my very LAST concern). France 10-15 years ago can lead as an example with a healthy rate of births, wealth and personal freedom.