Why the Administration Ignored Warning Signs
I suspect the Administration ignored warning signs that the ACA was going to be a quagmire because they were used to ignoring theoretical predictions, at least in this field. A typical debate on socialized medicine usually involves conservatives giving theoretical predictions that increased government involvement in health insurance would lead to no good which, in turn, could be met partly with anecdotes of health-insurance problems in the U.S. but mostly with the other side's trump card: It works overseas. As a result, advocates of increased government involvement in health insurance grew used to ignoring theoretical predictions about health insurance in general. (This is similar to the way a handful of slanted polls in the 2004 election turned into reasons to reject allegedly-skewed polls in 2012.) I'm sure that the people trying to implement the plan tried warning of disaster but were ignored on the grounds that “It works overseas.”
The libertarian response to claims that capitalist medicine is more expensive than socialist medicine is to point out that we have socialist medicine in the U.S. The difference is that the U.S. governments spread the same amount of money around apparently randomly. There's also the little matter that we allow private health care as well as public. This causes U.S. socialized medicine to look worse than socialized medicine in places where they can't compare it to anything else. I'm reminded of: When it's not being tested, it works, fact.