I think you underestimate how much the SCOTUS considers the interests involved in a case not just the law.
That's not saying that SCOTUS would invalidate software patents, just the opposite. The SCOTUS discussion I've heard have turned on how invalidating software patents would be a fairly short step to invalidating all patents and there are too many interests involved to let that happen.
The argument in the supreme court debate I remember was essentially "software parents as a generic are absurd but tossing them out completely would be an earthquake that we don't want to play so we need a line that would allow some but not all software patents". Nothing turning on technicalities in the overall approach even if the solution they'd grope for would likely turn on said technicalities.