I like this discussion but I am an extreme realist so all this talk of ideals and execution of ideals doesn't sit well. I'm going to try to spin things around and give a view that faces reality, hopefully without sperging too much.
There is an objective political reality. It is made up of borders, capitals, laws, institutions, armed forces, security apparatuses, loyalties, individuals, groups, a money economy, patronage schemes, and quantities of people. Although it is popular to see politics as an idealistic pursuit, "ideals" per se do not affect any of these directly.
Yet radicalism and radical movements exist. One of my medieval Muslim scholars says, "If all the quantities of wealth, properties, loyal men, soldiers, etc., could achieve absolutely anything, then no ruling house would ever have given up power to another." Only God/natural selection can do absolutely anything. Yet history is littered with fallen dynasties. So apparently we live in an intensely competitive world, and there is some opportunity to decide what we want from the political reality.
I guess what I want to ask is: does it make more sense to criticize the dodo bird, or the wildesbeest? The dodo is gone; nothing you can say can degrade it more than it has been already. God/nature already decided. And there are millions of wildebeest. It doesn't need you to talk it up, and if you need to coexist with it then criticism will be of no use.
So although it popular to say, "I don't think a communist system would work, and it appears that none have," it really isn't up to you. What has competition under natural selection shown us? Communist takeovers succeed sometimes. And notice that Vietnam resisted 3 foreign invasions. Cuba survived in an unstable neighborhood. Russia and China have not been conquered by Turks or seen their independence subverted by capitalists. Communism has proven to be a route from being a colony or a failed state to being viable and independent. Even North Korea, for all its suckiness, has maintained independence and self-sufficiency among powerful neighbors.
How can a state be radical, anyway? A state must be responsible for stability. Does it make sense to criticize it for not engaging in permanent revolution and ultimately killing itself? You would have to be a Trotskyite to do that sincerely. This would be like talking up the dodo.
Think about this: absolute rule is absurd. But rather than say, "that system is absurd," I'm suggesting, "that belief is absurd." It's impossible. You would have to be some superhuman, like Magneto and Professor X put together.
Notice that monarchies and patriarchies have ruled most people on earth since prehistory, yet "absolute monarchy" was not described until the 1600s in France. So why describe it that way? Every ruler depends on networks of supporters. The difference between "constitutional" and "absolute" forms is that the constitutional ruler has learned to let his power rest on regular (ie lawful, hierarchic) institutions that protect his rule as well as stand up to him; whereas the absolutist has not found the courage or interest to do this (he keeps an informal, irregular network).
This is true of fascism and communism as well. Everyone wants to say fascism and communism are about statism, but they can only be pursued through anarchy. No elite class will hand over their property to a revolutionary party, nor will some bourgeois parliament dissolve itself. The party will have to use its competitiveness and ability to act spontaneously to break the law and take over (ie anarchy). This is why Marx was openly anarchistic, although he is seen differently now. And how could Hitler have taken over while practicing statism? He didn't use state organs to pass legislation, file official decrees, and dispatch the police. He used his own personal following to terrorize, intimidate, and seize. This is anarchy, not statism.
My point is, rather see some "radical political system" as hypocritical or faulty, see it as a reflection of someone's political agenda at a particular moment. Do not see radicalism as an attempt to install the perfect system. That's an absurd proposition that makes everyone wrong. Radicalism is an attempt to overthrow what is in place. This makes a lot more sense and allows you to appreciate radical movements as things that can matter in reality.
tl;dr: Radicalism is a ruse; a pitch tailored to a particular end in particular circumstances.