I don't know how long primers keep, but I know that most of the primers sold today will be good long after the original buyer has "joined the quior invisible".
Because they keep so well if kept cool and dry, buying more than one could use in a single lifetime still makes sence if you can afford to do it. Like having a time machine to go back to buy hight test gasoline. If it would keep for a generation, you would buy 500,000 galons of it and bury it somewhere safe. You would then get in the time machine and come back to the present day burning up gas so good you can't even buy it anymore. Many people think that at some point reloading components will go the way of high test gasoline,
low cost factory muscle cars, and the freedom to own a M-14 or Thompson SMG. These people are willing to pay the price today because they believe that they could become priceless and completely unavailible in the future.
If you bought a Hemi Road Runner in 1969, and paid double the sticker price or more, you still made a great deal if you still owned the car today. If you died, and your son owns the car, it is probably worth more than his home.
The people have a unique perspective that looks into the future by knowing the past. They are willing to pay double the price for something today that will be priceless long after they are gone. If you are not willing to pay for it, stop complaining and wait like the rest of us to buy when the glut of stuff occurs.
If it never does, who was the wiser ?