As far as I know Obama-care did not totally replace medicare, there are some changes and cuts for those over 65 but they are not nearly as dramatic as the Obama-care changes to those under 65, Basic Medicare is already single payer, you cant get that much more regulated than it is.
But those in that first ‘pool’ that were born in that year 1956 would have it pretty tough because they would be alone. My proof? Because Ryan is going to those born in 1955 and before and say “you should support this reform because you are exempt from it.” just like Obama-care exempts some politically. If Obama's current plans for medicare are so much worse, why is the voting carrot to those born earlier that they will stay in it?
Of course, the media thinks that Obamacare with corrupt waivers and bloated bureaucracy is heaven on earth, but even without such a tilting of the game, it was predictable that voters would be nervous about whatever specific entitlement reform plan came out first.
Nothing really makes sense when those who are going to be swindled out of their money withheld from their pay specifically labeled for the entitlements that money is supposed to pay for, no matter which party gets its way. I don't think Obamacare is popular either. Both Obamacare and Ryancare play accounting tricks (for example, I think Ryancare assumes that the cost of medical care won't rise faster than inflation!). I think some voters will figure out how dishonest Obamacare was. But Obama was more diabolically clever than the GOP, and at this moment, Ryancare has taken some of the voters' wrath from Obamacare.