The justices can overturn the entire law, which would nullify many important health provisions that have already taken effect. For example, 2.5 million young adults under the age of 26 could lose insurance coverage through their parents' plans. Patient safety improvements that are already in effect would be reversed. Preventive care goes away and value based purchasing has just begun under ACA and could be reversed if the act is struck down.
The justices could strike down the individual mandate part of the law. After all, we are a free country and no-one should be forced to buy health insurance, right? Never mind that everyone is forced into Social Security and Medicare through payroll taxes and car insurance is mandated if you drive. Millions of people pay into Medicare and never use the service if they die before age 65 or are Christian Scientists or just hate doctors. No-one gets a rebate. Never mind that Emergency rooms and physicians are REQUIRED by law to treat uninsured people in the emergency room. No-one seems to mind about that mandate!
If they strike the mandate, they also nullify the provision for community rating and guaranteed insurance. There is no ACA without the mandate...something the GOP knows. Mitt Romney and Newt Gingrich were fervent supporters of the mandate before they ran for President. If the mandate is eliminated, it's back to the status quo.
The Republicans would like to make Medicare into a voucher program. That shifts the buck back to people who can least afford it and guarantees higher administrative costs. It essentially dismantles Medicare.
The Accountable Care Act is simply a start. It doesn't go far enough to reduce spiraling costs. It does address inequalities of care and helps reign in abuses. It is a start at shifting from "do more, despite the outcome", to focusing on value.
I support it and hope the Supreme Court does too.