Your assumption was correct as applied to me. I jumped to Plat, shouldn't have, and promptly booked ten nights during the Megabonus period that I would have given to Starwood otherwise.
I'm with you - no one is entitled to status - it is Marriott's discretion to give whatever they want, provided they don't promise something and then not follow through. *However*, you are missing a critical aspect here. In law, there is a concept called detrimental reliance (see
Promissory Estoppel subsection). Giving status and then taking it back, as long as the effect was just to make the person feel good and then let down, is one thing. Perfectly fine, innocent mistake, no harm done. However, what if that person spent $2,000 on hotel stays *purely* because of the status, relying on the status as a primary factor in the purchase decision, and then the status was retracted? Now they have spent $2,000 on rooms they perceived had a higher value. If Marriott refunds these with no question, then they make the person whole again, but if there were Advanced Stay rates or gift card purchases, they'll need to make policy exceptions to refund. However, what if the person purchased airfare to go along with the stays they paid for? Even if they room fee is refunded, they might be stuck with the airline tickets. Detrimental reliance. Marriott needs to be careful with this. Not quite a victimless crime nor a matter of whining customers who feel entitled when the situation involves spending real money based upon the reasonable assumption that the status was real.