I don't think you can claim that it's not needed just because a 'large percentage' of the population is well-served. The United States' population is over 313 million people according to the last census. Would you be okay with 3 million people dying or living in perpetual poverty because they don't have access to preventative/emergency care? Politicians in the US regularly make a fuss about things that affect less than 3 million people.
Now, you can argue that any government-run single payer system would have 'marginal benefit', but you'd have to work pretty hard to do worse than what we have now for those people.
Now, you can argue that any government-run single payer system would have 'marginal benefit', but you'd have to work pretty hard to do worse than what we have now for those people.