If SCOTUS upholds 0bamacare but strikes the Mandate they are WRITING LAW not ruling on the law because they’ll be adding a severability clause. Anything other than a total strike down is unacceptable because the Mandate is absolutely unconstitutional.
They've already shown that they're willing to rewrite law; consider the 2005 Kelo v. New London case.
In this instance the state argued that the "projections" of greater taxation revenue qualified for the "public use" requirement of the exercise of eminent domain despite that the public would not, in general, by using the seized property.
Another instance is Wickard v. Filburn, a root of much of the evil in our justice system, where the court decided that the interstate commerce clause applied to intrastate commerce because it impacted the overall market. (And that reasoning was again used in Raich, which expanded it to things never put on the market because it was illegal to sell.)