[ZenoArrow]

>"you and all your offsprings have two kids"

Two kids per couple isn't enough to cause population growth.

Two parents having two kids leads to the exact same number of people on the planet after the parents die.

We only see population growth with 3+ kids per couple, and that's becoming increasingly uncommon in developed countries.

[xiaoma]

It could. If lifespans were so long as to be nearly immortal (which may happen in our lifetimes), then the birth rate that would lead to stable population level would be one per couple. At that rate, a population of N would asymptotically approach 2N.

Absolute immortality isn't a plausible scenario, but it's important to consider that as longevity increases, the reproductive rate that leads to a stable population must go down.

[ZenoArrow]

Sure, at the point of signficant life extension we would need to set stricter limits on population size, but we're not quite there yet. Also, whilst there is some overlap, I think it's helpful to think of environmental damage and overpopulation as separate, as we'll have a hard enough time convincing people to change their daily habits without also asking them to give up on the idea on having multiple kids.