I was working for said company at the tail end of the partnership with WDW. The only thing I remember sporadically seeing was Mickey masks for the kids. (They could have had more earlier, but I wasn't employed with them during the partnership.)
In today's world, being the official airline of anything sadly doesn't extend to significant perks for the passenger. It means an airline plunked down money for advertising. You may get some bundle deals, but the discounts likely won't be stellar.
Airlines know what customers want -- who is going to get them from Point A to Point B the fastest and cheapest. Being official airline for a theme park is not conducive to their bottom line. Similar to the same reason why so many sponsors have left Disney. They don't profit from it.
Delta won't leave MCO. Although after losing Delta Express and the short lived Song, the growth has not been huge. But, they have added international routes. My best guess, I would not expect exponential growth as their home is only a hour flight away in the busiest airport in the world, ATL.