Probably the fact that these societies arose from violent revolutions doomed them. Revolutions tend to lead to authoritarian governments, because if you can take over the whole country with your army, you don't really need democracy to legitimize you.

I could easily cite Chile, Guatemala, Nicaragua, EL Salvador, Argentina, Bolivia, Paraguay, and countless other 20th century totalitarian regimes as instances where "capitalist experiments were doomed by greed and sociopathy." Very easily, given that these regimes were backed by American capitalist business interests. But realistically? When you're strong enough and ruthless enough to take over an entire country, you're probably more interested in power than in ideology, one way or another.

What if we are already living in a time where rich Western countries could experience post-scarcity if we just allocated resources and priorities differently?

If we're not there yet, what do we do when we are?

To me socialism is just an inevitability. We will eventually reach post-scarcity. I think if we can examine the current system and find so many ethical problems... It's worth asking ourselves what we could do to get to that ideal star trek society faster.