> Clearly it can't work though, right?

Obviously it's not clear to a lot of people that it can't work. I haven't thought much about it, but here are some responses to your hypotheticals.

> Someone has to decide what is fair, someone has to take things from people, someone has to decide who gets it

I don't think so. You could have the group decide what is fair and there need not be taking things from people if everyone is contributing willingly based on the agreed upon rules. Ownership is common, so control must also be common. As soon as you have a "someone" as you describe you've gone out of the realm of the communist ideology as I've understood it.

> Is it fair to take from someone who is more productive and give it to someone who is less productive?

Common ownership of the means of production and distribution doesn't imply that the distribution must be equal regardless of individual output. So it's back to the group deciding on how that works, since they are the common owners.

> A proportion of your most productive people who view the deciders and redistribution as unfair will want to leave for a society that better values their contributions, so now you need to place controls on them leaving

Even if the group decides on the distribution, you will have a curve representing individual production and another representing satisfaction with the rules. I think this is going to be the case in many ideologies. But this might only be a problem if you're presupposing other problems, like an egotistical psychopath "someone" at the top who insists on maximizing output from their workforce. Which is again way outside the communist ideology. At least, I didn't see anything that required forced compliance as part of the ideology. What if people who were on the strong disagree end of the curve could simply leave? Unfortunately in our world of countries we can't easily go somewhere that operates under rules we are more aligned with, but that's an implementation problem and in this discussion we're talking about the ideology itself, not problems with poor implementation. But we might be getting too close to the infantile view you mentioned. I suppose it's not unreasonable to say that espousing an ideology that can't work "in the real world" is infantile. But when Lennon had people imagining there were no countries and no religion, I don't think he was widely thought of as infantile. I think there is a certain type of person who believes that things can change for the better, even if the changes required are drastically different from what we have now. You could call this type of person a dreamer. Maybe being a dreamer is infantile. Anyway, I don't see how this is related specifically to communism.

> Another proportion of the highly productive and effective people will try to change the system to one that better values their contributions

To me this is also only a problem if other problems external to the ideology are presupposed. And is not communism specific. You've stated that the problems are basic, obvious principles about human behavior. I'm not capable of a decent attempt to refute that. We do have a pretty abysmal track record.

> so now you have to cease democracy and disallow political opposition

You definitely don't have to do that.