Up until not too long ago, every democracy relied on slavery, disenfranchised large parts of the population, and eventually ended up a dictatorship.
I don’t think that’s a fair comparison. Slavery was never an inherent part of democracy and democracy certainly didn’t rely on it. Ancient economies might have, but not their democratic systems of government. By contrast, communism does inherently call for the violent overthrow of existing governments in favor of a one-party transitional government that violently suppresses all others. Like I said, authoritarian rule is not an unintended consequence of communism—it is very much intended and seen as necessary.
Yes, communism has failed in many respects so far.* The reasons for that are complex, include active sabotage by anti-communist states, but anyone who doesn’t genuinely and critically reflect it’s failures is (probably) doomed to repeat those mistakes.
I don’t really think it’s that complex. Power corrupts, and absolute power corrupts absolutely. When you have a governmental system wherein multiple groups can check each other’s power levels, the system can self-stabilize (that’s not to say it always does, but it can at least). Communism, with it’s one-party system, has no checks and balances, and therefore is much more prone to succumbing to authoritarian rule.
You say we just haven’t given communism enough time to “get it right” yet; I say they’ve already gotten it “right” multiple times. China is communism working as intended.