Here’s another minimalist argument for invalidating the whole ACA: there would then be no need to reach the Spending Power issues raised by the Medicaid challenge. Either way the Court goes on that issue will be another big constitutional decision. The lesser course is to strike down the ACA solely because the mandate is constitutional and inseverable, and then decline to reach the Spending Power claim. Seriously, this would be a lot less constitutionally ambitious than reaching it.
UPDATE: I should clarify that the Court should rule on severability according to existing severability doctrine, although this doctrine requires the exercise of judicial judgment. My two posts on severability are offered in response to the nonlegal “prudential” considerations that are being urged upon the Court by others. I fully agree with Ilya’s refutation of these nonlegal considerations.