Libertarians act in the interest of preserving freedom in the long term, not maximizing personal wealth in the short term. That's one of the key differences between Libertarians and Anarcho-Capitalists. A Libertarian might stand up and say, look, our neighbours our hostile, I really think we ought to stockpile food, and if people are convinced (because it's true) then they'll do it. But the government will not say "we need to stockpile food so we're introducing a stockpile tax". Do you see the difference?
I completely agree that Libertarianism can't be retro-fitted onto any existing human society.
I do see the difference, I just happen to fancy my chances better under the system where the government imposes a stockpile tax than the system which leaves it up to the market -- even more so when, as I've mentioned in other posts, it's not about stockpiling, but about maintaining self-sufficiency in the face of an alternative that is more "efficient" economically.
Regarding your second paragraph, and completely independently, how do you propose we reach a libertarian society from the position we are now?
And how do Libertarians ensure that those they share a state with act "in the interest of preserving freedom in the long term, not maximizing personal wealth in the short term" in the particular cases where that may be non-rational?
They can't. That's the flaw. You basically have to start again with a bunch of Libertarians who raise their kids Libertarian (this is the easy bit) without it descending into a cult (this is the hard part).
The problem with retrofitting it is unless you already have the no-coercion culture and infrastructure in place, too many people acting in their personal short-term self-interest find coercion easier. Which doesn't mean mugging people in the street; it's as mild as "my vote for your handouts of their money".
I completely agree that Libertarianism can't be retro-fitted onto any existing human society.