As medical technology improves and the number of retirees increases, we could, in theory, keep lots of people around for a very long time, if only we tax everyone into poverty. I don't think people will accept that. So, some of the people who want the procedures, and could benefit from them, will die instead. Who does the picking-and-choosing is going to be an issue. If you think it will left up to individual MD's, you haven't factored in the lawyers.
The flaw in your argument is that the price of your "$3 million medical procedure" stays fixed at $3 million indefinitely. I still maintain that the only answer to the dilemma is the free market. Take heart transplants. The first one may have cost tens - if not hundreds - of millions of dollars, if one factors in the medical research leading up to it. Nowadays, you looking at about $250,000, well within the realm of any half decent insurance or co-op plan or even the procedes of one "Beef and Beer" night at the local church. Cutting edge procedures not involving the removal, storage, transportation and preparation of donor organs can cost half that. The same can be said for cancer and other treatments as they proceed from experimental to practically routine. The market has responded to insurance and hospital red tape by opening up commercial centers for many specialized procedures. Doctors now routinely advertise their laser vision treatments, "stand up" MRIs, and non-invasive colonoscopies on talk radio stations. I can't think of an example of a healthcare treatment or medicine that doesn't go down in cost with increased availability.
Your nightmare scenario is the stuff of socialized medicine run amok, and I think you meant it that way. It's an unavoidable flaw of capitalism that only the rich can afford cutting edge life saving medical treatments. Traditionally, charities have made up for the "inequities" that would otherwise put the poor at the medical disadvantage. The democrats know this. One of the things the Democrats tried to do "under the radar" during the push for HillaryCare is to make charity hospitals like Deborah ineligible from collecting insurance payments. If that doesn't give one a clue that the democrat party has anything but the well being of the American poor at heart, I don't know what would.
You don't get to hold the technology constant any more than I get to hold the price of any given technology constant. No matter what wonders become routine, there will always be some new wonder that costs $3 million, and there will always be people who need whatever it is.
In the past, doctors really could say, truthfully, "I'm sorry, nothing can be done." We are approaching an era where there will always be something that could be tried, the only brake on using it will be cost. This is a new thing for most humans to have to face. It will be gut-wrenching for many.