That's an important point. It's so hard to think of a better system, if you take the task seriously and actually think through all the consequences of each option.
As a result, as usual, the loud people that ignore all the details end up capturing everyone's imagination with a good story, and we stumble upon yet another century of nightmares.
Do you truly have a answer for a social architecture that is substantially better than a capitalist social democracy, flawed and compromised as it is? Because I really don't if I'm being honest with myself, and I am yet to hear one.
I think the problem I find with arguments that Capitalism is the best/least bad system tend to be that they start from a false premise, in my opinion. I have a friend who makes the joke all the time that any system of government works if people were just nice to each other, but he has a point. I often hear that "oh, communism doesn't work because humans are inherently selfish." That's true, if you believe that humans are inherently selfish, but my counter-point to that is asking how much of it is innate vs. how much of it is trained by our culture and reflects back in those communist attempts because the sudden change in social architecture didn't give enough time to 'train it out of' the culture.
Back to the thing my friend says - if you believe that communism doesn't work because humans are inherently selfish/greedy/etc, I'd say to you also that capitalism is currently not working _because humans are being selfish and greedy_ in a system _that explicitly rewards that_. Which, maybe is worse. Not as in the outcomes are worse immediately than Soviet Russia etc. but for the long-term trajectory of human society.
I don't pretend I have an answer for how we can get from point A (capitalist system) to point B (future space communism) in a way that slowly shifts human thinking towards mutual aid and collective action, but I think it's short-sighted to assume that the way humans act in a system that rewards greed/selfishness is innate.