Climate change isn't a problem of having too many people on earth, it's a problem of emitting too much CO2 per person. If we got rid of 90% of the world population, we would still be dealing with climate change, it would just be happening more slowly. Trying to fix climate change by reducing population is like trying to fix a leaky pipe by reducing the water pressure; yeah it might cause less water to leak but it doesn't solve the underlying problem.

One could argue that having fewer people on earth would still be a good thing because it would give us more time to solve the real problem of reducing CO2 per person; but it would also mean we'd have fewer scientists and engineers working to develop the sustainable technology needed to solve our problems. It's not clear if fewer people would lead to a better or worse end result, or if it would just get us to the same place more slowly.