I always hated everyone's (especially Apple's) skeumorphism. I wrote this http://nnife.com/inouveau.html about it, to me imitative digital design has always been horrible and it reminds me of the imitative architecture and furniture design that was popular about 120 years ago, like this article also mentions. It's such a dumb way of designing.
"Let's just make it look like this other completely different thing! It's familiar, so people will be drawn to it."
Pixels are flat. Screens are flat. Flat design is here to stay.
UI Design is a type of language and as such has to communicate a symbol with a meaning (or action in most cases).
If you can use a language element that your user already understands, why would you rule it out?
I also wonder why you compare Skeuomorphism with flat design when you can have a flat skeuomorphic design anyway.
And I'd also be careful with stating that any design trend is here to stay. Most design trends have been here before and will be back again in the future. It just happens that the flat-look is on trend.
I think web design is an art form. Like all art forms it is in a constant flux. We definitely are headed to a more flat era. However, I can not discount trends, it is their nature to change with the times.
Steve Coogan states in 24 Hour Party People, "the history of popular music is like a double helix." Everything is always a reaction the last trend. http://youtu.be/GhpezhJxUYU?t=44s
To bring up the affordance angle again, that doorway is extremely unsubtle about being a doorway, and gets high marks for usability, while the Le Corbusier building looks like a military pillbox with no way to enter or leave. All that ornamentation serves to guide the eye and give you a visual shorthand for what you're looking at, whereas the sharp textureless lines of modern objects and UIs makes me think it's all aspiring to be dazzle camouflage http://en.wikipedia.org/wiki/File:USS_West_Mahomet_(ID-3681)...
Besides, in the real world flat surfaces carry a gradient from their lightsource, truly uniformly lit, flat-texture surfaces are extremely rare and hard to orient visually.
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"Let's just make it look like this other completely different thing! It's familiar, so people will be drawn to it."
Pixels are flat. Screens are flat. Flat design is here to stay.