An excellent point.
Still, I think I would find it less fun to do new content to keep my old gear relevant than to do new content in order to get new gear. I think it would be frustrating to have to run a new raid, get the latest and greatest chest piece (one that I might want to hold onto for glamours btw) and then be forced to destroy it to keep my outdated chestpiece's stats up to code.
This could result in a situation where I'm raiding for an entire expansion just to keep bumping up the stats on a chestpiece I've been using for years, losing out on glamours the whole way because I want to maintain BiS.