One of the big reasons why healthcare is more expensive in the US is Americans get a lot more of it, particularly out-patient care (~150% of OECD average).
And other single payer countries certainly don't offer limitless levels of care. The gov't determines what it will pay for and nothing else. For Americans with generous insurance coverage, the care is much more limitless when you look at things like experimental treatments and the latest drugs.
One has to consider that many people in the US don't consume healthcare because they can't afford it (hence the lower life expectancy), whereas in other countries everybody who needs it can access it, with almost no limit.
So the part of GDP represented by healthcare in the US is much lower than it "should" be; or, to put it another way, there's a huge growth opportunity if one can lower prices.
Incumbent actors may prefer profits over volume, but they're vulnerable.