Because resources are finite. Sure, we haven’t tapped all the energy the sun shines on earth. But if we did, we would be doing the equivalent of painting the earth black, absorbing all possible light and making everything a scorched wasteland. Space travel enables a form of sustainable growth, but if you want to stick around here, the growth needs to stop eventually. I, and many people, believe we’re bumping up against the upper limit of humans the earth can support.
* child mortality is lower nowadays, therefore there is usually a temporary population boom in industrializing societies. This is because
* in preindustrialized societies, people are aware that their children are their primary means of economic support in old age. Pension schemes have eliminated this incentive in industrialized countries, but a collapse of such schemes will force people to reconsider this point. People might consider to re-prioritize children over other life goals.
Real activity on earth has much stricter limits. Humanity could expand into space, but growth on earth can't exceed various limits of commodities and ability to reject waste heat.
The growth is always good mindset is the same logic that would have you see a starving man fed being good and then concluding that shoving a firehose of glucose syrup down every person's throat must be better.
Growth is bounded. Resource use is bounded. Consumption levels in the worst polluting countries are past optimal for quality of life. The solution is to improve efficiency and reign in waste whilst uplifting those who don't yet have enough.
And how do you accomplish that economically? What technological progress helps stop continuing damage? What sort of policies to stop the damage are politically viable? There's 8 billion people who need to work, eat, move about, etc.
> But just building a bunch of nuclear power plants isn’t it
I would have been better than relying on coal plants, and would have helped electrify economies faster.
> Or better yet, figuring out how to end our reliance on a system that requires constant economic growth.
The world's population is still growing, and the developing world is still developing. How do you propose handling those two issues, while funding cleaner technologies, and finding an alternative economic system?
Why can't growth be sustainable with the right technologies? The sun emits vast amounts of energy. There's tons of resources in the solar system. Human civilization could exist for a very long time. We've hardly achieved everything achievable.