Insurance companies have to price their plans to turn a profit--they're not in it just to provide a non-profit service, like the government can. Further, while government programs can certainly generate waste when poorly designed or implemented, a single system has at least the opportunity to trim a lot of fat that exists in the existing system where every insurance company has to have their own infrastructure (like, actuaries), the operating costs of which need to be passed on to the consumer. Obviously, a Medicare-for-all would need plenty of infrastructure, but with a lot less duplication since it'd be a single entity. It would also probably be a lot less complex to figure out costs when the entire nation is a single pool, as opposed to each insurance company creating their own risk pools and trying to figure out how to price each pool.
I could imagine Medicare for everyone not necessarily working out great (again, design and implementation matters), but there is plenty of opportunity for it to reduce costs while still allowing people to get all important treatment covered (including preventative care which tends to reduce costs overall by reducing the number of people who end up with serious problems). We could even learn from what other countries, who have done universal health care, found to work, like charging extremely minimal co-pays (like $5-10) because attaching any monetary cost to something (even such small ones) tends to reduce frivolous visits while not deterring people who actually need a doctor's visit.