See above to understand why even asking this question is idealist.
I am going to have to recapture and reformulate what you are saying here to me so that I can be certain to understand it:
The idea of the working class creating an economic / political system fundamentally different to those produced by capitalism and moreover a system adhering to the ideas of communism, can not be discussed in its potential actual realizations because those realizations would just be ideas, at this point, and hence pointless.
As a consequence, to be a Communist means to assume that the various ideas of Communism would necessarily be realized, somehow, as soon as the working class is aware of them and starts to try to execute them.
I have to say that I find this POV not entirely satisfying or logically consistent, but since this is not a discussion thread, I will accept this as your answer (edit: assuming I got it right, of course! Please correct me if I misrepresented your position) and would like to ask a different question:
What I see implied in such a POV is a very strong agency of a very large group, i.e. the working class.
Why is such agency assumed? You may say due to the common interests of the working class, unifying it and enabling it to more or less act as one person/agent (hence: agency). But as soon as the working class has more or less abolished other classes and there is only one class left, why is it assumed that such agency persist beyond the stage of class struggle?
I am asking because to me a very large group of people engaging in intense cooperation on the basis of a common goal for an indefinite amount of time is a far from trivial thing to assume. If I, personally, look at human history, I see close-knit cooperation only under special circumstances and/or in fairly small groups. Whereas I see the opposite as the general status quo of large groups. Yet in communism, the whole of the working class, which ultimately will be the whole of humanity, is working together to do nothing but write history (am using this expression in accordance to what Marx said about history only really beginning when Communism arrived). So, I think it is fair to say, they will have to cooperate well enough to achieve the most monumental task imaginable. So they will have to cooperate very well. They will not just have to have great agency, but really really great agency as a group.
To repeat: Why is this assumed?
Or alternatively, why is my question wrong?