A few thoughts for the "Disney should do their own special thing..." camp:
Disney did do their own special version of a coffee shop. It was called the Fountainview Espresso and Bakery.
It opened in November of 1993 as an annex of the Sunrise Terrace in Epcot (the remainder of which was converted into Pasta Piazza by May of 1994.)
It was a full on "coffee shop" with bakery items and froofy drinks. It served Nescafe products as well as Disney food items. It had/has a show kitchen bakery in the rear and is entirely setup akin to a modern Starbucks but with two-sided ordering. There was even ridiculous "coffee" theme items covering the walls.
It never did well. It failed to attract guests and by 2001 it was on seasonal operation. The adjacent Pasta Piazza also failed to attract guests. Pasta Piazza became Character Connection as a test and due to the success of that test it was closed, torn down, and then finally became Character Spot.
Fountainview lingered on as a seasonal offering, until finally in 2007 they tore out the bakery case and put in ice cream as a menu offering. It has remained open as merely The Fountainview Cafe since then.
Disney's own conceptualization of a modern coffee shop failed to attract guests in one of the busiest theme parks in the world at one of the heaviest foot traffic areas. It simply wasn't good and it was a failure.
Now do any of us really think Starbucks is going to fail at that same location?
If you consider that factually, you can see why they're not going to attempt to make yet another Disney version of a Starbucks but rather just put a proper Starbucks offering in place at Fountainview and the Main Street Bakery.