(1) You'd eliminate the system of advantages and supports that cause employers to offer private insurance, which is where most people get their insurance from.
(2) You'd create a huge adverse selection problem --- the more effective/useful Medicare is, the fewer families will want to spent $24k/yr on private insurance, meaning the families left on private insurance have a reason to want it, meaning the composition of the risk pool would shift dramatically.
Like, if we ever did M4A, we'd probably end up with a widespread system of supplemental insurance; we already have it with Medicare! But that's not the same thing as keeping your existing plan.
If M4A plus supplemental insurance gives me about the same coverage I have now for a reduced total cost that sounds like a win to me. Even if it ends up costing me the same amount the net improvement from everyone having access to basic health care would still be a win.
