I have no reason to doubt that many people are fed up with the length and problems with the Marriott-SPG merger. It has been mishandled in a number of ways and customer service has been sorely lacking. I think former SPG elites noticed the customer service change the most because I never found Marriott Rewards folks to be particularly helpful whereas I always felt SPG customer service would go the extra mile especially as a Platinum member (even when I was Gold). While the merger between Marriott and SPG has been challenged I'm not sure it's as bad as the UA-Continental merger, at least yet. I feel Marriott understands the problems and challenges and things have changed especially with how they handled the database hack.
That said, and being a bit Machiavellian, if that many people have left Marriott to Hilton that is more reason to stick with Marriott because I suspect they'll take efforts to win people back and reward current loyalty. Maybe I'm wrong but I like to take a long view on loyalty and I've found sticking with UA-CO, even after all their problems, has paid off as the program has improved over the years.