You are correct in assuming that the majority of states implement low income Medicaid through commercial health care insurance companies.
Medicare, which is administered and run entirely by the federal government, does not use commercial health care insurance providers. If the proposed plans build on the current federal Medicare system, that would essentially eliminate the commercial health care insurance companies, by eliminating both employer funded plans and state administered Medicaid plans.
Or, if the new plans call for keeping employer funded health care, but moves Medicaid under federal administration, the current health care insurance companies would take a hit, but not a fatal one. In that scenario, one would have to wonder what the benefit to the taxpayer would be, as the federal government is not known for being proficient at controlling administrative costs.
If you are starting to think that candidates are making bold promises without knowledge of the cost or the difficulties involved... that is not without precedent in politics.