Just my two cents but the cost to manage 30,000 hotel rooms and villas is significant. Marriott (or whomever) would expect a lot in return, and Disney would have to make up that revenue elsewhere.
Yes, there are headaches involved with managing every segment of a business. But with those headaches come control and revenue maximization. Bring in someone else and you surrender control and $$$.
The hotels and their guests could stand to benefit. It's not something I'd be fearful of. I just don't see how Disney would view it as a win.