Actually the old "replace your mattress every 8 years" is an old, old motto. That started way back, I'm thinking in the 70's. I know I've heard it as long as I can remember. Not a new sales gimmick, just a revival of an old one.
OTOH I like to know that I'm not buying 10 year old tires. A few years old won't bother me that much since I'm one of those people that never gave the age of their tires a second thought until a few years ago.
As far as the gospel on tires, I'm not sure the the manufacturers' will necessarily give you the honest scoop, especially if it may be a bit detrimental to their product. :) I don't remember seeing Carlisle stating that their Chinese manufactured ST tires may not be the best choice of a heavy trailer. :)