There is no "promise", and the sooner everyone accepts this, the better.
If an elderly person has taken more in Medicare benefits than he has paid in (assuming a reasonable rate of interest on those payments), he is receiving welfare. Pure and simple.
Anybody who reaches 65 has paid into it their whole working life. The fact that some pay in yet die before they reach 65 means that there should be a pool of money left to redistribute among those who do reach 65. This means that on average everybody who reaches 65 might get more than they pay in.
Only folks who never worked, or worked very little, are getting a free ride.
You are right about there being no promise. I'm pretty sure that Medicare will be covered by the same Supreme Court ruling that says that the FICA taxes you pay for social security are simply a tax, and not necessarily related to what the government chooses to send to you in Social Security payments when you retire. If it wasn't included in that SC opinion, it will be if push comes to shove.
So, in the sense that Medicare was sold to the electorate, it was insurance.
In the sense of the Supreme Court ruling on Social Security, every payment is welfare.