Here’s a sentence from a science article by a writer for The New York Times. If anybody ought to know that the plural of millennium is millennia, he (and his editors) ought to. So this has to be deliberate.
And there is no doubt that thawing of the full depth of permafrost would take millenniums.
And I have seen plenty of places where the “singular” of criterion appears as criteria, its plural. Especially in computer-geek circles, certainly not an unintelligent crowd.
Pretty soon I won’t be able to harrumpf about it.