If the court strikes down Obamacare...

The rationale, after a fashion. After the argument, I was thinking along similar lines:

[H]ere's my devil's advocate argument about all this. No justice since Rehnquist has wanted to revisit the basic premise of the new deal revolution, which is essentially that Congress can do anything, but only four justices are willing to embrace that outcome, and so every time a case challenging congressional authority arrives, liberals can't believe it's serious (because they don't believe there are limits other than individual rights), and the conservative justices desperately search for a limiting principle that preserves the fiction that the new deal settlement doesn't make congress omnipotent (because they don't want to revisit the new deal settlement).

Now, I'm not sure that it's true that the new deal settlement essentially makes Congress omnipotent; certainly that is the proposition that Lopez denies. But in the absence of a limiting principle, it's hard to see how that isn't the upshot, so we are backed into the corner of accepting one of three outcomes: Accept Congressional omnipotence, impose (sometimes slightly artificial) limiting principles to reign in that power, or overrule Wickard et al.