Communism

Given that the environmental depredation of this planet is driven by […] can people explain why they believe that without capitalism

capitalist industry and commerce have been the driving force of the mass extinction of the last 500 years[0][1][2]. climate change didn't begin until the late 1800s with the rise of tycoons, and accelerated with mass production in the mid-1900s.

can people explain why they believe that without capitalism everyone would be […]

could be, not necessarily would. because a humanistic, socialised means of production would: allow for truly 'democratic' control over what is produced; remove nested interests and subsidies to overgrown polluting industries[3]; and make alternatives viable without the need to bend or break to top-down market pressures and monetary policy dictated by dragons.

I also assume they’re wearing hemp and have no interest in fashion.

capitalism has existed for less than 300 years. consumerism has existed for less than 100 years. when you have an economic system which emphasises the independent individual — simultaneously a motivator and a mere cog in the machine — and posits that the mere potential to own things is the source of value: buying wasteful, exotic, unnecessary shit is a way to define yourself and your status. it's called conspicuous consumption, and it happens from the micro to the macro in the lower and the upper classes, and there's top-down pressure to do so to keep currency current.

i recommend the documentary The Century of the Self for an overview of the commodification of identity and culture.

Keep in mind there are 8 billion people on this planet, so presumably they wouldn’t be having children either.

we are already producing enough food to sufficiently feed 1.5x the world population[4], and could continue to do so even within planetary boundaries[5].

i didn't cover everything here, because i recommend:

1. the book Less Is More.
2. familiarising yourself with the concept of the superstructure; it's a very helpful analytical tool.
3. going back to the last time you were on your malthusian debatebro bullshit and really trying to engage your imagination with much of the same arguments made there.

1. We know and can develop superior methods in agriculture, energy production and recycling material resources. The rapid transformation of the economy to make use of these superior methods will require state intervention and economic planning like never before seen in human history.

2. It is much easier to limit resource use when a certain segment of the population (the bourgeoise) are not consuming resources at 100s of times the rate of the ordinary person.

3. Eliminating advertising will reduce the pressure for overconsumption.

They would simply consume less and not be as driven to consume. Capitalism drives up the consumption to ridiculous levels, greed is not actually good. We could focus the economy on needs first and ensure it exists so people can still acquire goods and services in exchange for money so no one is working for nothing. But no more wealth accumulation into the stratosphere. There’s a lot that would need to change

That's an a-historical point of view. There have been several environmental catastrophes, including some causing massive climactic shifts introduced by prehistoric humans, some of them are documented in 1491, by Charles Mann. Poor farming practices, including some that have been practiced for thousands of years, are a huge factor in desertification. I completely agree that the rate and scale of environmental catastrophe is new, but the risk of it and tendency towards it is not. While I think capitalism is ABSOLUTELY the single greatest barrier to addressing the catastrophe, the scale and speed of that catastrophe could be just as easily tied to population growth as the emergence of capitalism.