Well IF she wins AND IF ObamaCare fails (no sign this is happening) then IF she has democratic majority in the house and senate, she could possibly TRY to enact a single payer system. Of course, first she would have to want a single payer system. One problem: she doesn't.
How so? Aetna was a large and growing company before Obamacare. It is less than 5% of their revenues now. Centene or Healthnet may have a problem, but none of the big 5. The numbers say the risk, if Obamacare implodes, is next to nothing.
Also, Obamacare is still doing fine. UnitedHealth may pull out, but that's fine. Although large, they were a late comer in the game and poorly prepared. Centene and Healthnet are making plenty off of Obamacare. United Health is just posturing. They're not going to leave a trillion dollar market.