It is true that the percentage of GDP the US is spending and will be spending in the future is not sustainable, but you're making an argument for universal healthcare. The reason costs are skyrocketing is that you have a privatized system that's (oddly) subsidized by the government. Patent laws give pharmaceutical companies monopolies on different types of drugs, while on the other hand the government is forbidden to negotiate with those companies on the price.
You also have huge overhead (31%) due to the inherent inefficiencies of running this kind of system through the private sector. For example, you need at least two doctors for each transaction - one to diagnose you and prescribe treatment and one working for the insurance company making sure that you actually need that treatment.
As for the argument about demand reduction, I agree that if you had much more demand, even reduced costs may make it unsustainable. However, in the real world that is demonstrably not happening. It simply isn't the case that universal healthcare is more expensive if we consider the evidence.