It was only because Roberts, not Kennedy, but Roberts a true conservative, sided with the other four, that the "ACA is a tax" became decisive.
And how did Roberts arrive at this decision? He quotes the 1995 Lopez case.
Small refresher course:
What the Lopez decision did was to invalidate any law banning the possession of guns near schools. How was this done? By citing precedence that this law challenged gun possession and presumably gun use. This activity, of owning a gun or using a gun, near school property was deemed too far attenuated from the stream of commerce for Congress to regulate.
So of course, anyone who was a gun activist celebrated the decision as a triumph of the Second Amendment, But the ruling was deeply troubling for liberals. Why? Because social conservatives saw the beginning of a new Golden Age that would seriously crimp federal power and future legal avenues to challenge laws and policies they deeply opposed like the Affordable Care Act.
So through the Supreme Court's stating that Congress does not have the ability to regulate the health insurance industry under the commerce clause, the Lopez conundrum surfaces. And perhaps "Lopez" will be used in the future to bite social progressives in the butt. These citations could well occur in the federal courts as well as in future Supreme Court rulings.
Does this mean that "Lopez" can be dragged out by conservatives to rule on further challenges to Medicaid, further challenges to Planned Parenthood, and the like? Time will tell.