That's always been my guess. I had a middle school teacher tell us about sneakers decades ago. My guess is that overtime mildew bacteria Etc grows into the fibers at a microscopic level and slowly breaks them down.

But that's just I guess. Whatever the reason is I can tell you that it works. I rotate between two pairs of shoes right now. Both Brands and models I've used for years. Individually one of them last about a year and the other about 2 years. I've had this two pairs of shoes for about 5 years now.