> The AMA _did_ lobby (I should have clarified that they no longer do this) to restrict medicare funding for residency
...twenty years ago, when we had the opposite problem. It still wasn't some act motivated by the desire to benefit doctors, even if that's the PR spin they used.
> At one point the AMA had about 75% of American doctors as members but has declined for various reasons (growth of specialty professional associations, change of employment in which many doctors have gone from private practices to hospitals which has accompanied a change in political objectives, etc.)
The move away from private practices was not the driving force behind the declining membership of the AMA. Quite bluntly, doctors stopped joining (unless they were forced to) because they did not support the AMA or its objectives. Why pay money to an organization that fights for causes you oppose?
Of course, this is only possible because (most) doctors are not required to be AMA members or pay membership fees if they choose not to, which is not true of people in most unions.
> The AMA probably does still serve as a professional association in the interest of some segment of doctors,
It does - it acts in the interest of the subset of doctors who are serving in administrative roles and are no longer practicing medicine full-time. That is to say, they advocate the interests of hospitals and payers, not practicing physicians.
> This is actually a good example of why professional associations can be inadequate,
It's not that they're "inadequate". It's that, in this case, they are literally fighting against the interests of the group they are (allegedly) advocating.
So really, the AMA is an argument against either professional associations or unions - doctors are unhappy with the AMA, and you certainly don't see them, by and large, advocating unionization in their practices en masse.