There is one argument that almost always seems to slip through these debates. And, for continuity, I'll use a modern Disney reference. In all it's Cheesy, synergy-marketing glory, the opening Trolley Show Song says it best:
"It's all pure magic wrapped up in Pixie Dust. Welcome to the place where dreams come true. A TIMELESS journey through lands of fantasy..."
Yes, they say all of them are lands of fantasy, so you can make the "everywhere is supposed to be fantasyland" argument. But, I don't think that is necessarily true. ALso, the Timeless part is what is lacking in some modern trends of WDW's MK. TL, when it opened, missed that point. It was designed to be a window into the future. But, by the sheer fact of reality, it became dated and... not timeless. If you look at the themes of the MK, they represent things that children, at least from the midlate century could relate to for their fantasies. Luckily, those were also things that their parents could relate to as well. Kids pretend to explore the jungles of the world; they pretended with Cowboys and Indians. They play pirates. They explore that spooky haunted mansion. They live out fairy tales. A science fiction-like future that was fantastical and based on enough reality without being tied down by it worked in that as well: Taking a rocket to the moon. Travelling through time. INteracting with aliens on a normal basis. This was the TIMELESS quality added to TL in the 1994 rehab.
Where we are going is somewhere different. We are timing ourselves much more than in the past. The Disney fantasy has taken over the timeless quality. That takes away from the "experience together" model that the Disney parks used to thrive off of. Everyone is ABLE to ride these attractions. Parents with small children can attempt to live vicariously through their childrens' enjoyment. But, what if people don't have these types of families? Monsters' Inc. and Stitch are part of the childhood, the fantasy of a small percentage of the current population. They aren't timeless. I really believe they will become dated just like a view of the future did in the early 1990s.
MK's approach seems to be about going to times and places not readily accessible to the average guest and about stories that exist solely there. Monsters' Inc. and Stitch were premised on a reality-based model even if they aren't "true". Once you start shoehorning in characters that are marketable, it is natural that guests may start questioning anything that involves them.