Skeumorphic features are defined by the fact that they are unnecessary carry-overs from older or different objects. They can be useful and style choices too, but a digital button does not need to indicate depth, whereas a mechanical button does, because it needs to protrude from the surface in order to be pressed.
A mechanical button absolutely does not need to protrude from the surface to be pressed - not only can it start at surface level and be pressed into the device, it can return to starting position and remain flush. Look at most microwaves - totally flat, presses in very slightly, returns to flush. The power button on the mac pro (that neglected tower!) does the same. Also, capacitive buttons exist which, although not strictly mechanical, have a similar behavior.
Protruding buttons are merely a user interface choice. They were never a functional requirement.
Not so slippery. "Tape reels" won't make sense to someone who has never seen a reel to reel. A knobby thing that protrudes, as if you could feel it if you ran your hand over the surface, transcends culture and applies to any normally functioning human being.
The difference between skeumorphism and visual affordance is the difference between "intuitive" and intuitive. One makes a reference to past experience, and is built on natural affordances. The other is not.
That said, I think what you're getting at is it might be impossible to completely eliminate skeumorphism. I agree with that. There's going to be at least a sliver left in almost any interface. That's just one of the byproducts of our having a culture.
An image of a tape reel is to recording audio as an icon of a floppy disk is to saving documents — a lazy visual shortcut, requiring a non-trivial semantic context. What is the percentage of humans alive today who have used a tape reel recorder?
The boundary between affordances and skeuomorphs is perfectly clear. Just ask yourself — does the form of the design element in question follow its function?
