Questions

Thre are 11 billion people on the planet. We're wrecking it for short term gain. We all know it.

We can't go backwards: reducing the population means all sorts of negative economic consequences making everyone poorer and the only way to do it is a via a single Dictatorial World Government everyone would hate, or a massive virus that kills say 99% of the population, thus delaying the wrecking of the planet by just 500 years or so as the population bounces back. And a medieval agricultural existence "in tune with the planet" just doesn't even bear thinking about. We're stuck with modernity.

Just knocking 10mph off the National Speed Limit or putting solar panels on your house isn't going to do it.
That's not the way. There has to be a technological way out of this downward spiral.

Expansion in to space and massive colonisation of other planets might help, but if 50% of the human population left (and they would inevitably be the bravest, the brightest and most capable) you would again run in to terrible economic and societal consequences: quite apart from anything else who would look after the old and infirm left behind?
And to what extent would the physical departure of 5 billion people affect the planet? Space elevators would be the only environmentally-acceptable way: 5 billion people on rockets would make the atmosphere uninhabitable.
And of course we would wreck those planets as well. We would become inter-planetary locusts.