Don't be too paranoid.
I'd be really surprised if Marriott announced these improvements and instituted devaluations at the same time without any announcement.
I think these are improvements EXACTLY along the lines of what I had most hoped for:
1. More bonus points for higher level elites, the current % bonus is a little too modest to be a real incentive to stay with Marriott more.
2. A good value reward opp. for stays of less than a week, right now all of the best value uses of points are for stays of 7 days. Busy people can't stay an entire week away that often.