According to the Medicare Chief Actuary Richard Foster, the effects of Obamacare on Medicare would lead to dramatic cuts in the reimbursement rates of physicians to levels even lower than that of Medicaid. This would lead to widespread shortages:

In the Office of the Actuary‟s April 22, 2010 memorandum on the estimated financial effects of the Affordable Care Act, we noted that by 2019 the update reductions would result in negative total facility margins for about 15 percent of hospitals, skilled nursing facilities, and home health agencies. This estimated percentage would continue to increase, reaching roughly 25 percent in 2030 and 40 percent by 2050. In practice, providers could not sustain continuing negative margins and, absent legislative changes, would have to withdraw from providing services to Medicare beneficiaries, merge with other provider groups, or shift substantial portions of Medicare costs to their non-Medicare, non-Medicaid payers.

And Democrats say that Ryan's plan would end Medicare as we know it? h/t:Avik Roy.