Yes, Congress can make us eat broccoli -- if . . .
This whole line of argument on a "limiting principle" is beyond ridiculous. The health provider and insurance businesses are interstate commerce personified, and for crying out loud, it's 15-frickin' percent of the entire Gross Domestic Product. For national welfare reasons, including massive costs the government itself incurs (at the expense of taxpayers) through Medicare, Medicaid and the VA, the total cost of our healthcare system needs to be brought under control. For many reasons, including the facts that millions who have no insurance nevertheless will be given healthcare at enormous expense to all the rest of us when they seek it because the public doesn't take too kindly to people dropping dead in gutters because they can't get treated; that providers are entitled to be paid when they provide services, and not have to eat those costs and try to pass them on to the rest of us; and that failure of people to seek treatment of what could be a dangerous communicable disease puts the entire population at risk, Congress and the President have determined that it is critical that we have nearly-universal health insurance.
The mandate does not exist in a vacuum, just because Congress and the President feel like imposing tyranny on the American people. It is the judgment of two branches of Congress and the President, which are the branches empowered to make such judgments on the wisdom of laws, that nearly universal insurance is now a national necessity and that the mandate is "necessary and proper" under section 8, clause 18 of the Constitution in order to ensure nearly universal coverage in a system based on private insurance. It is necessary, Congress and the President believe, because it is the only way of ensuring that such insurance is as inexpensive as it possibly can be by pooling the risks of everyone -- by not allowing those who are young and think they will be healthy to "free-ride" on the rest of us and leaving only those who are older or definitely not healthy to bear all the costs.
Your first limiting principle, therefore, is that the commerce that is being regulated by the Federal Government must be interstate commerce. If not, it is reserved to the states. Second, a specific requirement imposed upon interstate commerce must be "necessary and proper" to accomplishing the purpose of the legislation. Third, it must be passed by two branches of Congress and be signed by the President. So the answer is, yes, of course, the Federal Government can mandate that you eat broccoli if (1) it passes two branches of Congress (including a super-majority in the Senate) and is signed by the President; and (2) the regulation can meet the test of whether or not it is in interstate commerce, and (3) the regulation is necessary and proper to accomplishing the purposes of the legislation. The Supreme Court can review whether the latter two of those tests are met -- whether it is interstate commerce and whether the challenged regulation is "necessary and proper." But if all three of those tests are met, it is absolutely none of the Supreme Court's damned business whether a regulation mandating that everyone eat broccoli is a good idea or is stupid. There is a fourth limiting principle, too: people who think it is stupid can vote their representatives and the President out of office.
There are your limiting principles. End of story.
Scalia knows all this perfectly well. One can only hope he was playing devil's advocate. He knows perfectly well that the insurance mandate is 110% within the requirements of the Constitution -- that it involves regulation of interstate commerce and it is squarely within the authority granted to Congress to believe it is a "necessary and proper" requirement to make that regulation work. He also knows perfectly well that if those tests are met, and there is no question that they are, then the Supreme Court would be violating the Constitution itself to do anything to invalidate it. The Supreme Court has no authority to substitute its beliefs on the wisdom of legislation for Congress and the President.
Under the Constitution, Congress has the power, with the concurrence of the President, to regulate interstate commerce, period. Now we will get still one more test of whether Scalia or any of the other Republican-appointed Supreme Court justices are anything more than political hacks. That test was failed miserably in Bush v. Gore and Citizens United, both of which saw the Republican justices twist applicable legal principles beyond all recognition to achieve the political result they desired. This may well be strike-three-and-you're-out for the Supreme Court as a branch of government with any legitimacy.