At the risk of stating the obvious, there are two different features of the rewards program that are being conflated in this thread.
First is that rewards reservations that are booked and have certs attached will stay at the same points cost, even if the hotel category increases, or if the dates get designated as peak(?). When Marriott advises to book now to lock in rates, this is the feature they are promoting.
Second is Points Advance, which lets you book a stay before you have the points available. This is a nice feature of the program and by no means industry standard. For the vast majority of hotels not changing category, nothing has changed. There may be a one-time effect when peak pricing is rolled out, but going forward, peak/off dates will be known at time of booking.
Where the complaints are coming is from the intersection of the Venn diagram, from the subset of reservations taking advantage of both features. Where the hotel will be increasing categories, AND the member has insufficient points. The policy doesn't seem unreasonable, although the short notice may be.
My point is, those who are saying this policy will gut the Points Advance feature, and Marriott may as well stop offering it, are ignoring the large number of points advance bookings at properties not changing category. For those, the ability to secure the reservation before accumulating the points is a big plus of the program - especially since so many hotels do not offer last-room-availability for reward stays.