"The point is that developing countries will increase their per capita consumption regardless. If birth rate decreases with level of consumption/gdp, you want to accelerate into it, thereby limiting the peak total consumption."

Generally the accelerations coincide with one another. The only reason the population is increasing now is because some developments are being taken advantage of like food supply and medical care.

It depends if sooner is better. Many times it's easier to implement new technologies from scratch rather than revamp old infrastructure. If we rush implementation, then it can just be a waste of resources (look at sewage treatment in many parts of Africa that modeled it after the US).