That thought that continued growth is eventually unsustainable has been there since at least the 1840's. The core of Marxism is the idea that capitalism is the most effective method of increasing productive capacity, and that this is fantastic (the first chapter of the Communist Manifesto is fanboy-level praise for capitalism), until it runs out of room to grow into.
Whether or not you agree with the analysis, the core problem it posits - that capitalism will grow out of markets to expand into, and that once it does, it will fundamentally change the game - matters.
We can necessarily not have growth forever - if not before, eventually we run out of exploitable energy within our light-cone. In practice we'll hit limits far sooner that will cause competition to drive down production cost will be far harsher than it needs to be in an environment where you can compensate by expansion.
Marx thought that would happen "soon" - when capitalism has spread to the entire world. But it's a question when, not if, we hit a ceiling where further demand growth can't physically happen, and quite likely we'll reach a ceiling long before there where it's not happening fast enough to counter competitive pressure. The world population is expected to stabilise and start dropping - at least for a while - possibly as soon as towards the end of this century. What happens to growth then?
Whenever we reach a point where capitalism is no longer getting growth by growing into expanding markets, the question becomes whether capitalist competition will drive us to a level of automation that threatens to kill capitalism. Marx believed that would happen by causing crises of "overproduction", where improved efficiency would cause mass unemployment and by extension kneecap demand by taking away peoples incomes. Alternatively, whether we'll be able to as a society reign things in and ensure people share in enough of the improvements in efficiency for capitalism to survive - be it with UBI, or shortening working hours, or other means.
Because it's easy to forget that one of the "promises" of capitalist competitive pressure is to drive margins towards zero, because a company that can survive with lower margins can out-compete one that can't. And ultimately that means driving every cost that can be reduced down, and that means ultimately driving labour costs down as far as they can go...
I agree with a lot of what you just said, I think Marx was correct about the idea that capitalism requires the growth. I would state his thesis as follows: If you reward continued production of goods (which requires investment) with property, eventually you will run out of property to give away.
I believe in the 20th century, this conundrum was resolved in enormous growth of what we consider to be property assets. We have not only reached the limit of exploitation of natural resources on the planet, but also added intelectual property, military-industrial complexes, various financial instruments, healthcare, and as of late, we have monetized human attention and behavioral data. On top of that, we monetized human servitude, now lot of well-off people are in debt (at least with mortgages and student loans).
It's not so much that you run out of property, but that you run out of ability to expand your customer base fast enough to see fast growth, and the slower growth capitalists can get from external factors, such as new markets, the more incentives they have to look to internal factors to maintain their competitive advantage.
Ultimately that means addressing labour costs. For an individual capitalist it makes sense to drive labour costs towards zero. But if everyone does it, then their markets shrink unless there are external factors (e.g. UBI or similar) that counters the market-wide effects of their reduction in labor costs.
This is also why UBI is a "liberal" (in the classical, not US sense) policy, and not something the socialist left is very interested in - from a socialist point of view UBI is bread and circus to keep capitalism from imploding.
Whether or not you agree with the analysis, the core problem it posits - that capitalism will grow out of markets to expand into, and that once it does, it will fundamentally change the game - matters.
We can necessarily not have growth forever - if not before, eventually we run out of exploitable energy within our light-cone. In practice we'll hit limits far sooner that will cause competition to drive down production cost will be far harsher than it needs to be in an environment where you can compensate by expansion.
Marx thought that would happen "soon" - when capitalism has spread to the entire world. But it's a question when, not if, we hit a ceiling where further demand growth can't physically happen, and quite likely we'll reach a ceiling long before there where it's not happening fast enough to counter competitive pressure. The world population is expected to stabilise and start dropping - at least for a while - possibly as soon as towards the end of this century. What happens to growth then?
Whenever we reach a point where capitalism is no longer getting growth by growing into expanding markets, the question becomes whether capitalist competition will drive us to a level of automation that threatens to kill capitalism. Marx believed that would happen by causing crises of "overproduction", where improved efficiency would cause mass unemployment and by extension kneecap demand by taking away peoples incomes. Alternatively, whether we'll be able to as a society reign things in and ensure people share in enough of the improvements in efficiency for capitalism to survive - be it with UBI, or shortening working hours, or other means.
Because it's easy to forget that one of the "promises" of capitalist competitive pressure is to drive margins towards zero, because a company that can survive with lower margins can out-compete one that can't. And ultimately that means driving every cost that can be reduced down, and that means ultimately driving labour costs down as far as they can go...