The fact that the wealthiest people from other coutnries come here for medical care doesn't make ours the best system in teh eworld. By paying top dollar, they're getting access to care that you or I might have difficulty getting ourselves. And top-end, global-elite, best in the world type specialist account for only a very small percentage of our docs. Most of us get care that no one in their right mind would travel more than an hour for, let alone fly across the world.
And another thing. It's not just the US that is destination for overseas patients. Many now go to other parts of the world, including poorer countries such as Thailand and India, because the doctors are nearly as good in many cases, but the costs are dramatically less.
Having a few of the best specialiists in the world doesn't mean we have the best system. It would be like Serie A having two great teams (let's say Inter and AC Milan), who make the CL semis or finals every year, without fail. Often they play each other in the final. But below them, the league is in disarray. Several clubs can't pay their players wages, and their are pitch invasions nearly every week. The Italian teams in the UEFA get embarassed in teh first round, every year, by team fr even tiny countries like ALbania. The lower clubs pay very high wages, among the higest in teh world, but outside of AC and Inter, it's a mess.
In that situation...would it be fair to say that Serie A is the best league in the world? Sure, the best of the best want to go there (to AC and Inter, of course), but outside those two, the league itself is really quite weak. And that's our healthcare system. Great stuff at the top. Turmoil below. We're not the best league in the world. Not even close.