Optics do not "fade"
Vintage optics form oxide layer , or the balsalm can separate. This needs pro cleaning. In something like Vortex the tech is so settled even coating degradation would take a LONG time to happen[30 years] maybe less if getting abused. The worry is wear parts like hinge, focuser, eye cups and particularly rubber armour.
Even swarovski have had a major issue with rubber armour after reformulating for more environmentally friendly version. Probably lost long term customers. And everyone is aware of their great reputation.
Lots depends on type of user, guys in tropics experience issues with every brand as the humidity and temps are brutal on anything.