As far as wearing out, I suspect it is at least as much exposure to elements as riding. For example, one saddle I had new ~2001 I replaced in 2016. In between it went on a twelve month trip in 2001, a 10 month trip in 2007. Also shorter tours in-between and some use in commuting or other scenarios. I didn't always protect it on the rain. It wasn't completely worn out but was starting to develop some cracks and made sense to replace prior to a long trip in 2016.