What are you talking about? Humans have already irrevocably altered the natural ecosystem over the past 100k years. We have: killed off all the megafauna, domesticated thousands of plants, drastically changed the landscape through agriculture, literally changed the courses of rivers, diverted water all over the planet, etc. And that was all before the modern era.
Sure, this sounds reasonable, but it's really not when you look at the magnitudes of our impact over time. It's clear that the actions of the last 50-100 years or so are in extreme excess of what can be stabilised by the global weather and ecological systems. We've gone from about two thirds remaining natural wilderness in the world in the early 1900s to one third remaining today. The continued survival of most species, including ours, is very much in danger if we don't reverse a lot of this.
That assumes that population growth is somehow the primary cause of climate change and space expansion. The issue is lack of caring and technology, not population.
We don’t, however, have any idea how to keep humans alive after killing of all the other species. We appear to be totally reliant on some part of the natural world continuing to work, and we don’t know what part of it is required. Prudence suggests we be cautious about how much of it we decide to kill off.
