Communism is a classless post-scarcity society; it wasn't ever achieved, so how could it have failed?

If you mean USSR and the likes, those guys were claiming to "build" communism, with authoritarian socialism ("dictatorship of the proletariat") as the ostensibly required temporary stage that was supposed to be brief and kept getting indefinitely extended. However, none of them have ever claimed to achieve it.

It’s never been achieved because it would never work.

I mean the USSR was willing to murder people who opposed the idea and it still failed.

And China tried it and abandoned it out of failure true.

Your rationale is the “no true Scotsman” fallacy.

It's never been achieved because it was never really tried. To reiterate, what USSR and China had was not communism even according to themselves - I'm not even talking about impartial outside observers here! - so, pray tell, how is it the "no true Scotsman" fallacy?

Both China and the USSR conducted mass collectivation, land reform, commune models among other things to move away from capitalism to a communist model of public ownership. They even killed dissidents, violently oppressed public protests and starved their own people in the attempt.

And it was a massive failure.

It's a "no true Scotsman" fallacy because as long as attempts at moving to a communism model fail, supporters will just say "that wasn't true communism" ignoring the fact that if you can't actually create a functioning communist system that's indicative of failure of the model itself.

Like I said, everything that you have describe was not communism - it was a particular recipe for supposedly building it. And yes, that recipe was a massive failure, and few people on the left believe that it could have ever been anything but.

But here we're talking about communism naturally arising out of technological post-scarcity. What does that have to do with Soviet model of "public" (actually, state) ownership, or killing dissidents, or violently oppressing anyone?

If we can have communism in the solar punk sense without surveillance and all the scarcity, then I am in. If not, then that idea better dies in hell.

The idea is that it's not something that we can really choose to try or not. If/when we get to the stage where enough is produced to allow post-scarcity even in theory - i.e. the technology and the logistical science is there - it implies an extreme degree of automation. But if we have that, how would a traditional capitalist economy even work, if all that automation is concentrated in relatively few hands as it is now?

It works today because the owners of capital need workers to make that capital produce something useful, and even with all the economic rent fleeced from them, those workers still retain enough to then buy the products (if not their own, then that produced by somebody else). This enables money to cycle in the economy. But the more things are automated, the more people are unemployed, and - in capitalism - effectively excluded from the productive economy. So it's a question of how many people you can so exclude before the whole thing crumbles down.

Post-scarcity will never happen. How do you pay for the latest medical technology unless you have the money to develop it? How about other technology?

Maybe capitalism will be replaced at some point but you and I will be long dead before that ever happens.

Capitalism may well become as dirty of a word as Communism or Socialism in our lifetimes. When food, healthcare & shelter become unaffordable to enough people, this legal fiction, too, will fail into untold suffering and revolt. History does indeed rhyme.