Michael Kinsley argues in Bloomberg View 1) that striking down the mandate would call into question decades of commerce-clause jurisprudence, making even the Civil Rights Act of 1964 vulnerable and 2) that nobody argued the mandate was unconstitutional until after Obamacare passed.
As for the first argument: Well, maybe, but I suspect that any opinion striking down the mandate would be pretty careful to distinguish precedents. The legal briefs against the mandate are not generally asking for the last few decades of commerce-clause precedents to be overruled; they’re arguing that upholding the mandate would go beyond those precedents.
As for the second argument: The factual premise is false, although it is certainly true that the constitutionality of the law became more of an issue once the law was enacted. But leave that aside. If the argument that “nobody told us the law was unconstitutional at the time” has any bearing on the question of whether the law is unconstitutional, then what do we make of the many, many laws that the Supreme Court has struck down even though they were on the books for decades without anyone’s questioning their constitutionality? Most of the laws nullified by Roe v. Wade had been in place for decades, as had the laws invalidated by Lawrence v. Texas. As far as I know, nobody questioned the constitutionality of these laws for most of their existence.
(Kinsley, by the way, says “it is generally considered” that the Court was right in Lawrence, which also seems to me to be a false statement.)