So, given that capitalism has demonstrably failed the world over, what's your beef with communism again?
I don't think this is the point.
It's a matter of recognizing that communist politics have indeed been practiced, and concluding from there that the very possibility of a fundamental change is completely lacking.
As I've stated, this represents a wrong take on the program and theory (ince OP clearly stated that he/she wishes to deal with the argument that the idea has been tested and failed) since it completely neglects the actual conditions of its implementation or practice.
But to insist on the fact that capitalism fails is to neglect that most of such arguments presuppose that capitalism actually doesn't fail since social reproduction on the basis of capital still persists. It seems that the criterion is whether a social, economic and political order manages to reproduce itself, and the situation with so called communism is that it failed in its reproduction. That's a fundamental reality we need to consider very seriously, and not by resorting to hand waving (not that I imply you're doing it, but I can't help but notice that many people resort to such "debating strategies").
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“The possibility of securing for every member of society, by means of socialized production, an existence not only fully sufficient materially, and becoming day by day more full, but an existence guaranteeing to all the free development and exercise of their physical and mental faculties – this possibility is now for the first time here, but it is here.” Friedrich Engels
"The proletariat is its struggle; and its struggles have to this day not led it beyond class society, but deeper into it." Friends of the Classless Society
"Your life is survived by your deeds" - Steve von Till