I don't think Marriott will devalue Platinum, although the Platinum Status may be devalued by an influx of new Elites.
Judging from a $650 AF, I really don't think Marriott would be dumb enough to strip the benefits away. This is a HEFTY AF.
We need to realize that we are in a bubble. The amount of people interested in these cards are much less than what we assume there to be.