> My parents were upper middle class, but there's no way they paid enough into Medicare to cover the amount of medical care they've used just since retiring (last 5 years or so).

It's a common misconception that the money we all pay into Medicare is going into a generational savings account, so we all need to "pull our weight" to make sure there's enough in there by the time we start withdrawing from it. Medicare is a pay-as-you-go system though – the money we pay into Medicare is being used by the people who are on Medicare right now, and when we're old enough to be using Medicare, it will be paid by the taxes of the young people who are working at that time.

Your parents shouldn't feel guilty for using more of the Medicare resources than they feel they paid in. That money was used up years ago by people who needed it back then.