Rubber yoga mats, just like, sneakers or shoes will turn dry if not used for long. I've not done yoga for more than a year. So, my less than two-year-old yoga mat is slowly gathering dust on the outside of the cover. It is slowly sticking together when rolled up, and the rubber is flaking too when scraped.
I hope it will get totally flaked so that I can find no use of it and discard it. What I fear is if the mat is only partially 'unusable', this will tempt me to hoard it until it is really not functional.