I'm not sure *what* is behind the degradation of rubber and elastic parts at HP-- but I can tell you it is not just limited to 41cx readers.
The 67 and 97 readers, the big "D" shaped tire on the 9100a, and capstan drive tape machines such as the 9144a are some of those I can directly testify have turned "mushy" with time.
I have also seen this pathology on the older Colorado QIC-style tape drives, though the cases I saw were all attributed by the service tech we used to "excessive shuttling"-- where the tape used might have a block that can't be read properly, so the drive might forward and reverse the tape over and over to try to read it. It was guessed that these machines, doing unattended backup and verify, were "shuttling" all night. The equivalent of "driving the tires off" a car.
I am darn sure that my 9100a reader was never used that way; ditto, I would think, for the card reader on my 97. It seems just age did it in those cases. Heat is often blamed, but I am not so sure about that. My car has PLENTY of rubber that never has melted, though it has lived most of its life in this part of the southern U.S. we affectionately call Hell. On the other hand, you would expect that an office wallflower like the 97 would neither be exposed to, nor generate, enough heat to make a difference to its parts. Why would heat particularly be a problem in the cardreader?
Other possibilities people have mentioned:
1. Air. Some rubber products oxidize, and thus breakdown. Surely you know that your reader was only intended for use in a complete vacuum? :-)
2. Some microbes attack and eventually eat natural-based rubber. Witness the foam pads on your eight-tracks. (The "Andromeda Strain" theory)
3. Aerosols and vapors from solvents are attracted particularly to this rubber. Your cologne was too strong.
4. HP used a supplier with an inferior formulation of rubber (this in comparison with TI, who never seems to have this happen)
5. The lack of use of the parts is the problem-- because they get hardly any exercise, the "soft" polymers dry out of the piece, leaving porous compounds that disintegrate on use. (This is the "Armor-All" theory; supposedly, adding polymers in will keep these parts "new").
I don't know what validity can be assigned to any of this. (My bet is on #4). I DO know that it's a problem, for sure.