Medicare is barred from using its economies of scale in order to bargain down the prices for drugs, which would save it and those that use Medicare lots of money. The corrupt politicians also have made it very hard to import cheaper drugs and they and their corporate backers go to places like the WTO and have a seat at the table to undermine generic drug production in other countries, create a very protectionist intellectual property rights regime and make drug re-importation really hard in this country (and this despite the fact that we the taxpayers pay a large percentage of R & D costs for their damn drugs). There was a time in which Medicare didn't exist. Do you think life for the elderly was better then? If so, do tell, and provide some data too.
It is a fact that the overhead in traditional Medicare is a fraction of private health insurance, even with the ACA, and again, that is with Medicare not being allowed to bargain down the price of drugs. Single payer systems in other countries have far less waste and overhead, are cheaper and we spend more as a percentage of our GDP on healthcare than any other country on the entire planet according to World Bank data. Given why there is so much waste in this system, there really isn't anything that can be done unless you move towards single payer. The more plans, the more choice there is, the more plans that hospitals and other providers have to juggle. The complexity of this system results in the need for a massive administrative apparatus, and it results in lots of the system having to hire financial professionals to manage the complexity. Nothing can be done about that, or the marketing and lobbying costs, the massive executive pay, the profits, among other things. Single payer systems aren't perfect, but they cover everyone, they have less waste and they don't have the massive social costs we do in regards to thousands dying each year from a lack of access to healthcare, countless people being locked into their jobs and the large amount of bankruptcies that result from healthcare costs.
By the way, when people say Medicare for all, they are obviously talking about expanding Medicare to fill in the gaps in existing Medicare, as is the case with single payer systems elsewhere. In Canada, however, single payer doesn't cover dental care and dental care is important as far as person's health, so that might be another gap to fill.
Fight the Power? LOL!