Everything you wrote could be true, but I don't see how it is any different from the attempts of certain Hilton and Starwood properties to wiggle out of their award requirements by limiting the number of standard rooms at a property. Devaluation can come in certain forms, namely:

1. Point expiration - Marriott and Hyatt do not do this, but SPG and Hilton have it.
2. Category creep - Every chain has this, and in my experience, it has been somewhat less at Marriott than at Hilton (try finding opportunity or category 1 properties).
3. Large scale changes in programs - I would put Marriott's proposed changes here.

Marriott has become everyone's favorite target since this announcement, but I can assure you that devaluation is an ongoing process at all hotel chains. Is Marriott the worst place to earn awards? It depends what type of awards you are after, what your stay patterns are, and what you value in an award program.

People complain about the new category 8, but the biggest devaluation is the reduction of less points for longer stays at one property. Going from category 7 to 8 is only an additional 5,000 points/night. This is about a 14 % increase. When Hilton created their Waldorf=Astoria properties, the point increase was 50 % (except for low season, which is less than a third of the year).

Devaluation is a constant in award programs and most paper currencies. Hotel programs are better than airline programs because there is extra capacity for redemption. I am sure this upcoming change will move some people to other chains. Will everyone leave? No, and not everyone should for most devaluations (I will exclude US Air's gutting of their elite program from this statement.).