Medicare Part D isn't a private plan. Private companies are intermediaries--and make profit--but just because they are surrogates doesn't mean it is private. You have an odd definition of "private." If it were, then there is no need to have the government involved and there is no need for the government to spend money on it.
These surveys that show senior contentment with the program are interesting--if not bogus. I know no one who is content with the program, especially the 30 or so retail pharmacists I know personally who, without exception, are taking loans to survive or are on the edge of bankruptcy.
The program bought Florida in 2004, but it took away any pretense of the Republican party being the party of competence.
Yes, you do have a point. Gov't does subsidize the drug plans, and they do make the rules. But they aren't doing any more than employers do - they contract with medical suppliers to provide health care to a common group of people (in this case Seniors, instead of employees) and they provide some subsidy of the cost.
The agreement is between the insured and the medical co., and not between the insured and the gov't.
It isn't a stretch to think that this path will lead to a number of medical cos. who can provide ins. to Seniors at reasonable cost, and that socialized medicine (gov't provided) would go the way of Canada and Britain, among others.