We are a planet trying to find the resources for eight billion human beings and we already know the backbone of our global agriculture solution is an unsustainable fossil-fuel-based phosphate cycle.
People choosing to not have replacement-number of children is one of the more humane ways to balance the cold equations for subsequent generations.
Hard to predict. The resulting chaos could cause outsized environmental damage. On the other hand, the environmental damage caused by wars tends to be short-term (in contrast to the damage caused by, say, fossil-fuel-based energy production and agriculture, which is perpetually-increasing damage to feed the needs of a society with a stable population and status quo).
But you're making an excellent point. I'm certainly not advocating for societal collapse. I, for one, think the risk of such is a bit overblown (societies tend to adapt, not implode, even in the face of demographic turbulence).
Besides, automation's supposed to make up for much of the labor shortfalls anyway.