If you move to a Medicare for All policy it would shift employer health care costs to the government. Which they would never be able to financially support especially with the challenges of an ageing population and excessive debt. And so when they say it saves $450b it's not taxpayers. It's mostly employers.
There is a reason that countries that started with M4A e.g. Australia, UK have all transitioned to a public-private mix because it's simply not sustainable otherwise.