Earlier quoted context omitted.
Have you ever bought an old cardboard tube of salt? The last time I did was in a tropical location. Somehow over the years it had acquired the scent of wet rot, and the relentless cycles of humidity had effectively recrystalized the contents into a rock. Luckily the packaging had started to biodegrade, too, so it was easy enough to scrape off the fiber fuzz and hammer down the inner salt rocks into something useful.…
How do you know this happened as a result of the sellby, as opposed to being improperly stored? The package could get wet at any point.
My broader point, I guess, is that the appropriate date to sell something by depends so heavily on factors unknowable at production time that it’s hard to guess. But also that apparently-shelf-stable goods can in fact degrade.
And I guess that it’s kind of weird and unnatural the way modern industrial packaging/processing have trained us to expect that a given product should be interchangeable with any other until an arbitrary date.