The only way this would apply is if they pre-paid for their hotel. And I'm guessing that anything spent on airfare is irrelevant, because while the erroneously upgraded status might have guided the choice of hotel, it is unlikely to have caused someone to take a trip that s/he would not otherwise have taken.

In the (I'm guessing) extremely rare cases in which someone actually pre-paid her/his reservation, the loss is pretty small. As upgrades are not guaranteed, the only benefits s/he could have been counting on are the points earned bonus, CL access if available at the location, and the plat arrival gift. So...if anyone actually booked a pre-paid room during their week of ersatz platness, it wouldn't be too hard for Marriott to make her/him whole. And I'm guessing we could count the number of people this applies to on one hand.