If a generation is 25 years, there 7.9 billion people on Earth, and the replacement rate is 1.0, then humans will disappear in about 800 years.
If we enforced a 1.0 replacement rate for two generations, the global population would decrease by 75%, leaving 1.9 billion people in play. This is the global population in 1919. If we go three generations, we could get down to 985ish million prople.
That would be amazing for our climate goals and would be considered ethical and humane by most.