I do not see why it would not work in the US if it is doing great for France. I think that it well help a great amount to the poor and the middle class and even bring more balance to the country as a whole. The US should do it.
Though I would not agree with the implementation of a socialized system, I do believe that a more socialized system would be less open to corruption like our current system, even before the full implementation Obamacare. Healthcare outcomes would decrease with the French system, but it would be more sustainable that our crony system.
It will not and shouldn't because we are fine how we are right now. If we change we will find some defaults and that will trigger a change in economy. If we try there is still a 50 50 chance that we can be successful but why take the risk after such development of the economy. A comparison to Monoply (the board game) if you try to do something like buying lots of house and go bankrupt it is because you didn't do it properly and it will take you some time to pay it back.