Despite the fact that the Senate GOP's healthcare plan is dead in the water, the following is still relevant. The Republican leadership still plans to vote on a straight repeal of the Affordable Care Act (with the actual effect possibly to be postponed until shortly after the midterm election in 2018), and that implies canceling the expansion of Medicaid. This is the main objection raised by governors of both parties at the U.S. National Governors Association’s summer meeting in Providence, Rhode Island. This three-minute report from Sunday's PBS News Hour explains some of their concerns.
Another problem is that repealing the Affordable Care Act, whether it goes into effect immediately or in the future, is necessarily going to destabilize healthcare markets. It will also cause huge problems for individuals contemplating early retirement, starting a business, or otherwise thinking about leaving a job that gives them health insurance. The Affordable Care Act assures them they can get a policy on the individual market. Without the Affordable Care Act they're taking a gamble.
The ACA has its problems, but every major proposed so far has problems as well, and very possibly worse ones. The only exception would be a true expansion of Medicare to everyone, including Medicare Advantage. That way people would still be able to obtain private insurance if the preferred and they would also be able to buy Medigap policies. That would preserve consumer choice and the private health insurance industry rather than force everyone into a pure single-payer system. (Single-payer has its virtues, but the loss of choice would scare a lot of people.)