Sagging Voice Coils?

You might notice; that article, and it's accompanying remedies, addresses downward facing woofers only(as in some subwoofers). Many of which have heavy cones and/or long excursion(some double) coils. The effect of that type of sag would displace the voice coil, from it's centered/at rest position.

Step Six is speaking of the spider sagging toward the magnet(read it again). Something that I ran into frequently, with older woofers, when in the reconing business. Just the opposite of what one would get, in a downward firing woofer. That would occur quite often, in the humid Florida climate. The moisture would weaken the sizing(stiffening agent) in the spider, and allow the pleats to distort/warp. On occasion; I'd run into a woofer, with a pleated cloth surround, that had the same problem, and a rubbing voice coil. It's not hard to see how the same thing might happen, do to age.

I'm just talking from my personal experience, as a speaker reconer. The article YOU cited, was referring to the spider sagging towards the magnet structure(not the frame), and states exactly that. Like I said, I don't doubt that could occur, do to age, but- my encounters have been a result of damage caused by Florida's high humidity.