While your math is correct it also contains a false assumption about how much land is needed: What do you think about Netherlands? It seems to me they had a go

While your math is correct it also contains a false assumption about how much land is needed: What do you think about Netherlands? It seems to me they had a good way to use the space while keeping it green and also caring about nature. Earth can support at least twice the current number of people with a better organisation while also disrupting less the natural habitat. What makes it hard to achieve this is our focus on nationalities and countries instead of thinking about a global society combined with our greed and our individual/centrism way to think about the world as if the only way forward is us vs the forest or us vs the wild animals ... we should focus more on how to embed into nature and not how to change it to make it work for us.