Most large companies presently self-insure when it comes to their employees’ health care plans. Frequently they hire erstwhile insurance companies to administer their plans. They do this for a reason: they save money relative to what they would otherwise spend.
How payments for a “Medicare for All” system would be structured is important. In all likelihood it would cause those companies to pay more than they do presently and many people who presently have insurance will end up paying more.
The U. S. health care system is very complex. There isn’t just a big pile of money labelled “health care spending”. There are tens of thousands or millions of little piles and how the money is shifted from pile to pile will make the difference between a program that can be passed and one that gets its proponents thrown out of office.