The problem is that Medicare can't control costs too much better than private insurers or, as you see from the article above, doctors will simply abandon Medicare. In a world where there's only Medicare and Medicare decides to control costs, doctors can either take the pay cut or stop being doctors. And as we see from other countries, lots of people want to be doctors, even if being a doctor doesn't make you particularly wealthy. But in a world where Medicare is just one of many payers and Medicare decides to control costs, doctors can simply stop taking Medicare patients and a lot of legislators will lose their jobs.