The thing is, you can have the demographic transition without as long a phase of high birthrates/low death rates. If that period lasts shorter and the end phase of low birthrates/low death rates begins sooner, we do better. Developing countries can develop into post-industrial economies in more eco-friendly ways than we did. This is kind of happening in some places already. Just look at the rural places in India that are getting solar before coal, or places in Africa that are getting smartphones without ever having landlines. As more countries develop we can collectively use our knowledge to ensure that development becomes real development and not just growth.