Nothing ever completely replaces everything.

That's kind of my point. Anarchy is more an idealistic aspiration than a practical system. We'll get to anarcho-communism eventually, it's practically a certainty, but that's probably a century or two in the future. We need a lot of fundamental cultural and material shifts before the average individual will embrace the concepts, and there aren't any shortcuts in geopolitics.

Have you ever actually implemented a system that replaces an existing one?

A government? No.

I didn't say a government, that was not the question I asked: a club function, a group project, a large event, a project team; any coordination of people for the purpose of performing some task or making some decision. The basic principles are similar to a government, though difficulties multiply with scale. But the difficulties intrinsic to organizing humans are present in some form at every scale. If you've ever organized humans, you'll understand the difficulties I'm referring to. If you haven't, this topic might be beyond your expertise.

So very frequently, the replacement is ambitious and idealistic, but it's not until implementation that the flaws become apparent.

Which is why you need to build it.

Again, the hand-waving. "The answer is to just give it a shot!" That completely ignores the fact that politics is inherently social and emotional. A poorly implemented experiment will sour the public on the concept, and ultimately do more harm than good.

Any political strategy without an eye for optics and effective persuasion is useless. Action for action's sake is literally one of Eco's features of fascism. We didn't need to fall for that trap. We are an experiment, we need a carefully constructed, detailed, tested plan to generate buy-in. Politics without buy-in is nothing but theory.

'Jefferson went further, proposing that the nation adopt an entirely new charter every two decades. A constitution “naturally expires at the end of 19 years,” he wrote to James Madison in 1789. “If it be enforced longer, it is an act of force, and not of right.”'

A new charter, a new direction for the system, a new iteration of the system with revised mandates. I don't think he would have anticipated total rejection of the basic model.

The implementation is probably the thing that's debated most. The thing about the left is they mostly all agree on what they want, but they don't agree on how. Some want a revolution, because they think that'll create a better foundation. Some want gradual change to existing systems. Etc. You have an idea in your head that you want to be true, and you assume it must be true, but you don't actually want to find any knowledge to (dis)prove it.

I agree that this is the general condition of the left, and a tad ironic for you to say. I've explored most of the clades of political theory, particularly on the left. I've read a lot, and agreed with a lot, but I have too much experience organizing people to accept the rose-colored glasses endemic to anarchist theory. Again, maybe in a century or two, but there's too much intermediate growth as a species necessary to actually operate that way.

It just doesn't offer much of substance which isn't addressed more thoroughly elsewhere. Even the central concept of "hierarchy" is difficult to pin down. I've heard definitions so liberal that modern western democracies technically count as anarchic since representatives are voted in, and definitions so conservative that they couldn't even convene a public works task force.

And that's really the central point. With every logistical difficulty, anarchism either hand-waves "The people will agree cooperatively", or introduces a provisional acceptable hierarchy that looks an awful lot like existing structures at scale. It's just not a strong foundation at this stage of human civilization.