Population is not the primary problem.

Some people have a massive ecological footprint. For instance, somebody who uses a private jet ~daily.

Other people have a negative ecological footprint: their actions either actively improve the ecology of the world or act to reduce the ecological footprint of others.

The difference between the two is many orders of magnitude. A world-wide population of 100 million daily private jet users likely has a larger ecological impact than 10 billion low-impact humans.

Population change takes decades or centuries for the ecological impact to happen.

Shifts in the ecological footprint of significant populations can cause shifts over a small number of years.