Degrowth seems to me like a vision of the future that assumes that humans should remain slaves of their environment and not become masters of it. If humans are to become the dominant species in the universe, degrowth will not help.
From my reading of degrowth works the answer is: because now the global resource use induced by growth have accumulated enourmous negative externality costs, costs that will nedlessly increase further unless counteracted by degrowth policy. Food shortages, deadly heatwaves, property destruction. Direct dire costs to human health and welfare and rippling secondary effects. For example how would a scenario with hundreds of millions of climate refugees play out on top of current global political tensions?
I see the degrowth view not as "we should never ever have growth again" but rather the current modes of growth have very tangible imminent (on an astronomical timescale) negative effects that we must deal with right now. If we fail to step up to the current challenge of provisioning health and welfare for beings on this planet how likely are we to manage an all universe encompassing expansion without it ending in catastrophe on the largest scale? You could see the climate change challenge, and the degrowth plan to tackle it, as a necessary step to get our house in order, before expanding out.
Us attempting to expand into and shaping the universe is quite a task - so much could go right or wrong. I think Toby Ord's The Precipice[0] has a very useful framework for thinking about such risks.
As far as dire scenarios are concerned, I struggle with them as usually the realities play out differently. Arguably in part because people do consider such scenarios.
Seems to me it is often portrayed that way by opponents strawmanning it or by media simply very lossily compressing it into a few sentences/seconds chunks. But I'm not sure how many of the proponents really have that view. Even in cases where degrowth writers have "simple life forever!" type phrasings my impression is that that's not really the focus of their work and they're not really strongly wedded to such a vision for the long term. Their core focus is the imminent (as in 100 years) negative externalities, not the long future after that.
I think Jason Hickel is an interesting case. I've learned a lot from reading both his work and that of his critics. His "What does degrowth mean? A few points of clarification" is a good intro to his thinking[0]. I think very little there is really incompatible with expansion into space in the longer term.
[0] https://www.tandfonline.com/doi/full/10.1080/14747731.2020.1...
This is precisely where we were when people first started raising alarm bells about greenhouse gases.
The thermal emissivity of the night side of earth is only one of many limits we are running head first into. Almost all scarce resources, many abundant ones, and even abstract concepts like the growth of classical computation are all hitting limits where they only scale linearly or worse with energy input.
Growing into the solar system is largely irrelevant because it's a constant increase vs an exponential and will not be meaningfully coupled to earth's economy without energy expendature on earth proportional to trade volume. And there's no compelling reason to hinge everything on the assumption that the second law of thermodynamics and speed of light are both false so nothing outside the solar system is relevant to those living inside it unless we overturn one or both first.
All of this to avoid reexamining the blind faith assumption that we have to give an exponentially growing share of the pie to those who have the biggest share. Growth is only necessary for survival of the 99.99% if every year they give 2% of their wealth to the 0.01%
Edit: "materially" in the sense of stuff. Some data from a nearby star could still be highly valuable, but exchanging stuff over lightyears might rarely be efficient.
You ask why now? The answer is because we've already overshot the ability to do everything we do sustainably with the resources available. We need to get off the treadmill now and hope like hell technology and the available buffers are enough to limit the harm to 'merely' hundreds of millions dying.
Also, for example, innovation from growing economies elsewhere can reach back to earth. Expensive stuff that relies on growth can continue to happen and the results shared.
Carrying capacity of the solar system is huge, so quite some growth to be captured there. Once it is exhausted, growth for earth and the solar system might slow down because information will have significant travel time. And we haven't even exhausted earth and earth will become uninhabitable ultimately anyway so terminal sustainability is not truly needed.
This entails growth in most primary industries also ending, as well as a large reduction in many.
This is incompatible with a system where people starve and everything breaks whenever growth goes below 2%.
"Stop giving everything to the people who already have stuff so we need the pie to grow at an exponential rate just to keep the share that goes to 99.99% of the world constant" isn't a hard concept.
GHG is bad because it produces single digit watts of forcing (captured solar heat that would otherwise escape).
Direct thermal forcing from either capturing sunlight (making the surface darker) or emitting heat from nuclear derived (or in short term, fossil fuels before they run out) sources is just as catastrophic, but further off.
We can happily replace our current energy needs with nuclear and renewable, but we're just back at square one if we don't end growth.