It's not just Disney either - it's happening everywhere, including Vegas where "detheming" is in.
I think they're all taking the same page from current marketing theory (including movies) - it's not enough to have a singular vision or story anymore because you inevitably cut out chunks of your audience when you do. That's why many movies now-a-days (especially the blockbusters) have numerous subplots to appeal to different audience types. But you have to stay under the 2 hour length time to keep audience churn going so you inevitably end up with watered down plots that appeal to the lowest common denominator to maximize audience potential.
Detheming of the resorts is the same thing - If you do anything in one particular fashion (the beancounters reason) then you're harming the potential of 100% occupancy so best milquetoast the whole thing for the broadest appeal. This cuts costs in other ways as well because you don't get unique furniture anymore and can just buy off the mass market or do a one-off run. (Unlike, say, the original furnishings in the Grand Floridian which were custom made just for the resort)
Expanding that to Vegas - the Luxor started off with an Egyptian theme and even themed rooms - which has been so watered down over the years that internally they don't refer to it as "pyramid" anymore but "the atrium". The rooms were recently "renovated" as well and filled with the Bellagio's OLD furnishings which, in the Bellagio, were the beginning of "detheming" there as they were 'deluxe' but non-descript furnishings compared to the original furnishings which matched the color motifs and italian stylings of the Bellagio's original themes.
If anything the entire concept of a "resort", both at Disney and in Vegas, seems to have been lost and is now treated as a "motel 8 with a built in shopping mall"
In Vegas, I totally get it. They went through this phase where they thought they could market themselves as a good family destination while somehow keeping their reputation as adult playground capital of the world. Clearly, they've since given up on that misguided approach.
It's been a good long while since I've been out there (because I now have a young child and have no intention of dragging him through a casino to get to a pool or having to explain what those "collector-cards" someone on the street was happy to hand him a fist-full of were about) but being a M Life member, I still get the offers.
You mention Excalibur and I have to mention, I've gotten no-strings attached offers to stay for free there just... because... which tells me something. I suppose it's sort of hard to totally de-theme that particular resort but the last time I was out there, it appeared to be geared toward the barely over 21 crowd who basically couldn't afford to stay anywhere better and didn't mind that their hotel looked like a kid's pizza and game place from the outside as long as they were close to everything else.
The last time I stayed at Luxor, I don't recall anything seeming Egyptian in my room but maybe I just didn't notice.
Of course, all of the newer stuff for what? About the last twenty years? Has been built from the ground up with no theme.
In the case of Disney, though, I think they need to be careful. Maybe in isolation they could get away with it but doing that along with other resort choices in recent years seems to be really improving the value proposition of nearby off-property resorts.
Somewhere further back, someone mentioned Marriott World Center. In my opinion, that's a beautiful hotel/resort that also happens to be very close. With rooms starting at $119 a night (right this moment - I just checked), I don't know how Disney plans to compete with that in the contemporary de-themed future where everyone pays to park and resort fees* on top of room rates are the norm both in and outside of Disney - especially when they're getting rid of things like Magic Express and messing with their extra magic hours.
I mean, $119 is cheaper than Disney's
value resorts most of the time and those things are basically gussied up motel rooms.
For those who have never been there, let me assure you, World Center is no motel.
As standard contemporary hotels go, Disney certainly doesn't seem to want to invest in competing on true luxury so I don't understand what they expect the draw to be.
That said, I think the Polly is a long ways away from de-themeing on that kind of level but it
does seem to be the new approach to anything they're thinking about building, anymore.
*I know Disney hasn't implemented resort fees
yet but we all saw the survey questions about it so we know it's something they're trying to figure out how to tack on.