Three most common quotes by Socialists:
Of course those are all what's known as the "no true Scotsman" (appeal to purity) fallacy. They have to do it, because any example of real Socialism, ended poorly (North Korean, USSR, China, Venezuela, Cuba, Vietnam, Cambodia, and so on). When you can't defend your position, put up an impossible standard to excuse all the failures. But you know anyone doing this either can't accept reality or are frauds knowing your examples are valid so they change the standard/goalpost, thus they can be written off as intellectual children, not yet ready to have an adult discussion on the topic.
Some argue that the best way to combat this argument is to jujitsu them, and use it back on them. "Well real Capitalism has never been tried", and make them defend against their own impossible standard.  But to me, that's the whole, "I know you are, but what am I" form of arguing. While it's true that the argument that true capitalism hasn't been tried... and the closer we got, the better things worked -- it's an argument that should stand on it's own. Not as a form of counter-attack/distraction for the Socialists.
So in the end this is a rhetorical trick that's not going to persuade a Socialist. And you can use it as a cheap shot that may score you points with the lurkers, who laugh at the irony of the point (it cuts both ways). But persuasion is educating on why Socialism always leads to failed states, not by changing the topic.
In the end, Socialism can only work for short periods of time, and if you have nothing to compare it to. It is basically the idea that you should punish those who work the hardest and succeed the most, and reward those that do the least. When you first eat the rich, and redistribute what is theirs, it is tasty, and the lower people are better off. Then over time, innovation slows, bureaucracy takes over, performance declines as the disincentives take root. Then your culture learns the futility of effort, the rewards in cheating the system, resentment at the system and others for their miserable existence, and you get a malaise. Atlas shrugs. The hard working leave, or go where they will be rewarded: criminal enterprise or party politics. Both are parasites on innovation and growth, and the economy may perform in spite of it, but it performs less than the alternative. This is why every socialist system collapses, the only question is how slow will that society mature and grow past it?
The same with why the devolve into autocracy and bureaucracy. In a diverse society, not everyone is going to agree. Socialism can't exist if even a small percent of people are opting out and keeping what they earn for themselves. Others would see this and copy it. So they must destroy/imprison those individuals, and fight human nature (the desire to improve your station), at all costs. The same, they can talk about idealism about everyone voting on everything and nobody owning anything -- but both are untenable and impractical. So they setup bureaucracies, and have the state own everything and just loan it to others. So there's still property for me (the state), just not for thee (the individual). There must be centralized authority, because individuals can't agree on everything and they need the proxy of authority. So the idea of the collective must become the party versus everyone else (individuals that resist).