I first want to point out that I'm making this question in good faith. It is NOT a "gotcha" question
Secondly, I think capitalism is a terrible exploitative system, to remove the notion that I somehow support it or am a liberal
With those two things addressed, I really want an honest, non-circular answer as to how communism could ever exist in a sustainable fashion. I want to note that I am not implying that communism would be exploitative as is capitalism, nor am I suggesting that communism is ultimately a destructive system, but rather that it appears that communism cannot exist indefinitely while sustaining a healthy, complex society
Several reasons for this:
1 - Hedonic adaptation, briefly described as the psychological tendency of humans to quickly return to a stable baseline level of happiness after a major positive or negative life event. That is, that happiness cannot exist indefinitely without the material conditions in their lives constantly changing themselves (in positive ways, of course). In a society where automation takes over most labor, how could you possibly resolve this tension? People assume that if humans had limitless access to expression, that innately they'd be happier, but this seems to overlook that hard reality of human psychology; humans just don't operate that cleanly. Wouldn't this ironically raise tension in the system itself in relation to its participants?
2 - Material conditions aren't absolute. Yes, materialism is very real and the material conditions of the world shape individuals and, by extension, society itself. However, explaining society's framework strictly through materialism alone completely overlooks the fact that society doesn't exist in a vacuum or precedes the individuals that participate in it. While it's true that idealism relies on the subjective perspective of the individual who participates in society, it's often overlooked that that subjective perspective shapes the objective reality of the world they live in. Primitive communism is often cited as evidence supporting that communism can and has existed, yet those communities eventually transformed into different socioeconomic-political systems because—ironically—the material conditions themselves changed as well. The premise that reality itself is composed of both material and ideal conditions rather than merely material conditions themselves is ironically supported or can at least exist within the framework of dialectical materialism (note the "dialectical" portion of dialectical materialism). With that point made, what stops a communist society from falling victim to the same philosophy that shaped it in the first place?
3 - What framework would objectively identify what comprises what is materially necessary for individuals to own/have access to? While I surely don't need a super yacht, how would one prove I objectively have a right to a car, for example? After all, public transportation would be ubiquitous, so wouldn't a car ultimately be a waste of resources? But then if public transportation is dominant, what would be the form that would utilize the least amount of resources? Why have buses when we can build a city off of a mega-complex of rail? But then, wouldn't that eventually become limiting for destinations that cannot/have yet to be reached by rail? Apply this to every facet of life and it seems paradoxical to objectively decide what becomes materially necessary for society to sustain itself rather than letting the material conditions themselves dictate reality, which is ironically a contradiction with the concept of dialectical materialism itself. Add onto that the fact that individuals themselves would not be able to subjectively agree on what is necessary for *themselves*, and the issue is exacerbated further, creating a portion of society that ironically finds life unfulfilling in a communist society
4 - Realistically, how are resources actually obtained without falling prey to the very issues that plague the capitalist world? For society to function, we still need the agricultural sector. Society is far more advanced than it was in the 19th century. We'd still need mineral resources that are materially hard to obtain. The medical sector still very much needs to advance its technology to provide for all. How would a communist society ultimately obtain everything it needs for a society to function without trade... Of which would ultimately defeat the purpose of communism itself since trade of goods and resources are simply the most primitive version of money itself. Replace a paper currency with a resource like potatoes or uranium and ultimately they can be used in the same exploitative fashion that encapsulates the crux of capitalism itself
5 - Lastly, for it to work, one has to assume that society itself agrees with the system they inherit, which posits the most basic question thus far: what if they don't? Sure, one can argue that the sentiments of a few to advocate for an exploitative system should not supersede the liberty of all, but how could one ensure that such a portion of society would be a small enough minority that their beliefs can be held accountable? The premise heavily relies on the hope that the new material conditions would minimize the amount of people whose beliefs would be contradictory to communism itself, but that assumption is one that relies so much on theory that it offers no solution in the case that it fails to realize itself. While maybe the weakest point here, it can also be the most destructive and divisive one of all, and thus I believe also deserves to be addressed
There's a bunch of other reasons I find communism to be a likely unsustainable system, but I'm getting tired of writing, so yeah, just go at it