It's a Hyatt Explorist->MGM Gold match. Frankly Explorist is a joke midlevel status, pretty comparable to Marriott Gold, below Hilton Gold. It's basically extra points and bottled water at checkin.

I have no idea what MGM and Marriott have planned but I don't think it's entirely unreasonable that MGM might give Gold at Marriott Platinum. A lot of people are not going to drop $650 for a credit card, and if they are, they aren't all Marriott FT "maximizers". Many of them are going to be people who might do 25 nights a year anyway.