Well, the short of it, as I believe I've gathered, is that our current system is helplessly expensive and wasteful. Therefore he sees no hope in merely changing who pays for it.
Medicare generally pays less than private insurance though so it should certainly act as a mechanism to make it less expensive.
What's odd to me is that conservatives simultaneously claim that Medicare for all won't help make our system cheaper while simultaneously arguing that Medicare pays so little that doctors will quit and we'll be left with shortages, etc. Logically both cannot be true. Okay it's not odd to me, it's just another excuse to do nothing because opposing an expansion of Medicare is the goal and they will use any argument that is convenient in the moment.