I don't believe that's true; unused gold coins expire at the end of the same month during which you acquired them, one year later. (I believe silver/platinum coins are similar, but only last six months)
So for example if you acquired 10 gold coins during July 2020 (it doesn't matter when or how many purchases or how spread they are through the month), all ten of those coins would expire simultaneously at the end of July 2021, while any coins you acquired later would remain. It's not a radioactive decay at all; it's a simple expiration date on each individual coin, like you get on coupons or other incentive programs.
The coins expire for the same reason that coupons expire, airline loyalty program miles expire, etc -- they're a liability on the company's balance sheet which potentially could build up infinitely if there wasn't some limit to how long they needed to be tracked and redeemable.
So for example if you acquired 10 gold coins during July 2020 (it doesn't matter when or how many purchases or how spread they are through the month), all ten of those coins would expire simultaneously at the end of July 2021, while any coins you acquired later would remain. It's not a radioactive decay at all; it's a simple expiration date on each individual coin, like you get on coupons or other incentive programs.
The coins expire for the same reason that coupons expire, airline loyalty program miles expire, etc -- they're a liability on the company's balance sheet which potentially could build up infinitely if there wasn't some limit to how long they needed to be tracked and redeemable.