You acknowledge that the vast majority of the evidence points towards universal healthcare systems being more economically efficient in your first sentence. You then proceed to make an argument that basically asserts the US is a special case and that a universal care system wouldn't "behave" as it does everywhere else and reduce costs, but instead increase them. I see no basis for this assumption and I want to challenge you to demonstrate why you should be granted it.
I also want to challenge you on your implied definition of "useful" or "efficient". You say you would favor a universal system if you were to see convincing evidence that it would reduce costs. In your argument, I at least, read an implicit assumption that cost ought to be the deciding factor. I find that point of view detestable. For corporations, whose sole purpose is profits, it makes sense to think of efficiency that way. You and I, however, are people, and shouldn't have values that put resource acquisition higher than human well-being. You're free to disagree, but doing so would convince me that we have very little common basis for dialog.
In the last part of your comment you give France as an example of a country that will suffer dire economic repercussions for having run an unsustainable universal healthcare system. I haven't double-checked your references, but I'm happy to grant you, for the sake of argument, that the data is on your side. You've chosen to ignore the vast majority of strong economies with universal healthcare. You've also failed to mention that whatever problems France may have in the future, they pale in comparison to the budgetary problems the US is facing due to the privatized healthcare system it is currently running. References forthcoming should you ask...