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You aren't suggesting it shouldn't be bought in such copious quantities because it's a bad product, but rather because it benefits others to reduce demand for it, right?
It's actually a terrible example for the point I'm driving at, because I think it's precisely the kind of thing minimalists should increase their consumption of: high-quality low-quantity substitutes for mass-produced crap.
But, yes, my point is that by cutting back my consumption, others benefit (although, not Oshi). In practice, we can more easily see how others benefit because Hodl Butter is actually way underpriced and they're always out of stock. When I cut back, Oshi earns just as much and someone other than me is able to get a jar.
Is your position that an increasing cultural shift toward minimalism would be good for the world because people will be happier with a lower standard of living?
Not at all. Go back to my 30,000ft overview. I continue producing just as much and reduce my consumption, thereby leaving more for everyone else. Standard of living go up.
if there is less demand for lots of stuff, but it doesn't make the sellers of that stuff poorer because the demand for their inputs has gone down as well, doesn't it just mean the standard of living went down for everyone?
This is why I detoured into talking about why supply curves are upward sloping. As demand shifts down, we enter more efficient production processes. You raised several objections to that claim, which can occur, but are not the norm.
That means, on net, income losses are more than offset by cost of living decreases. The most impacted producers will suffer net losses, but go back to that "rest of humanity" point to see that those resources are better deployed elsewhere. The bulk of producers will be better off. Also, @SimpleStacker made a good point about what producer surplus is.
we might say that we have all the consumerism and stuff and airplanes and other things we spend money on because in aggregate people want those things.
Want + can afford, yes. The second part is also necessary and is where productivity comes in.
Minimalists are willingly doing with less, which leaves more for those who still want all that stuff and, because the new minimalists are no longer bidding the prices up, there are people who become able to afford things they were only able to want previously.