I don't own Marriott, but have exchanged into them via II 5 times. I am wondering why Marriott owners are unhappy with II. I thought the Marriott exchanges I got were fair trades for all involved parties. At face value, many trades appear to be down-trades for Marriott owners (higher purchase price and higher MF), but I don't believe this is true. Marriott tends to offer surplus inventory mostly in slower seasons to II. In addition, II exchangers in general get the undesireable locations within Marriott resorts. Some Marriotts will try to honor room requests for exchangers in low season, others (e.g. Ocean Pointe) will give you an undesireable room location even in slow season.
Marriott resorts seem to me to be built with a relatively large number of undesireable rooms (bad location/no view). In addition, there seem to a large number of owners fighting for decent rooms. The leftovers that exchangers get don't have that high a trade value. In other words, a lower ranked resort in II can compare very favorably with a higher ranked Marriott that will give you a less-desireable room.
If an exchange is created with multiple high end hotel chains, prime inventory will have to be surrendered by Marriott and the other brands. The inventory currently given to II exchangers will not be adequate. I don't think Marriott owners who are already desperately fighting for good rooms will be happy seeing prime rooms disappear to exchangers. Otherwise, the new exchange will be similar to II, handling surplus and less-desireable inventory. Marriott owners will still think that they are down-trading (e.g. trading an OF Marriott for a Hyatt less-desireable room).