The best alternative to Obamacare that has been put forward so far is that of the 2017 Project. (I’ve commented on it here before.) Now the Center for Health and Economy has estimated what its effects would be. The results: More Americans would have private insurance coverage, premiums would be lower, patients would have more access to doctors, and the deficit would be lower. There would be no individual mandate, no employer mandate, no essential benefits package, and so on.
Obamacare would, however, cover 6 million more people than the alternative, thanks to its Medicaid expansion. I imagine that gap could be closed, though, if the alternative included auto-enrollment.