1) Ongoing Disapproval Of The LawFor more details on each points, read the full text of "Four Ways Obamacare Could Still Fail".
2) States Declining To Expand Medicaid
3) States Refusing To Build Insurance Marketplaces
4) Nullification Of The Medicare Cost-Cutting Board
Also, if/when the current system fails, it's not clear what will replace it. We could move in the direction of more freedom or towards a government-run "single payer" system. That decision will still be up to us.