Holy crap! You've got some seriously toxic stuff there. Primer residue contains a high concentration of lead styphnate. If your cases are coming out clean, all that crap has to go somewhere (and the dryer sheets aren't removing all of it).
I don't know how much you reload, but if you're running the same media for years, you're probably handling an insane amount of poison. I figure for the price of the corncob media ($20 for 50lbs) I can afford to change it before it becomes too poisonous.
Keep in mind that it still cleans OK when it's old - that's not why I change it.
I suspect you're right.