If a law is unconstitutional, it does not matter how long it takes for that fact to be determined, it does not suddenly become constitutional by default.
Ostensibly, but realistically, if the law were unconstitutional, it stands to reason that someone would have sued over it almost three decades ago, so the burden of proof required to argue against its constitutionality is basically insurmountably huge after so many years of no one doing so.
The only plausible grounds for arguing unconstitutionality after so long would be if there were new information that did not exist at the time, e.g. if you were able to collect statistics from a generation of people living under a school segregation law showing that separate but equal doesn't work, or if some new group of people were born who were somehow affected by some law who had some protected characteristic that did not exist at the time of the law's creation, or... something else new that could not have been determined at the time.
Absent that sort of pivotal change in reality or information, decades of presumption of constitutionality represents a high enough bar that such a challenge would almost inevitably fail. There's really no reason to not apply laches after such a long period of time, in the absence of new evidence, a new class of aggrieved party that didn't exist at the time the law was passed, or some other substantive change in reality that would make the constitutionality suddenly worth calling into question, where substantive change in reality is something much larger and more significant than a particular party winning the White House and deciding to throw their weight around in an effort to smash everything they don't like.
More to the point, in a reasonable world, the courts should have applied punitive damages for wasting the courts' time with such a frivolous, laches-doomed, decades-of-unchallenged-presumption-of-constitutionality-doomed legal challenge. It should be genuinely *masively* expensive to pursue a case like this about something that is not exigent. The courts have enough on their hands dealing with all the constitutional challenges from the actions of the current government without having their time wasted litigating the constitutionality of laws passed before a decent percentage of eligible voters were even born, and the fact that even a "win" would have been nothing more than a procedural unconstitutionality that limits how government does something rather trivial, rather than a meaningful unconstitutionality that truly deprives anyone of rights or creates any sort of grave societal harm makes the case doubly appallingly. Everyone involved in bringing that suit should be ashamed of themselves.
This was a waste of the courts' time. And although in some ways, it is nice that the courts shot down their absurd argument once and for all, it was still a waste of the courts' time to bother doing so.
The worst part of it, though, was realizing that the opposition's position, if taken to its logical conclusion, would also make it illegal for the IRS to accept credit card payments, accept direct withdrawals from banks, and lots of other similar situations where a private business ends up collecting a tax on the government's behalf. It would nationally eliminate red light cameras. It would nationally eliminate private parking enforcement contractors. And so on. The sheer number of things that would have to be rolled back and rethought if the other side of this case had won is mind-boggling to such a degree that a modicum of common sense makes it clear that their argument had no merit.
Estoppel by laches would have been too good for this case.