This is why I'm a big fan of the IX suggestion; Bard AF could have Enhanced Ballad, and if we were just doing as XI did, you'd either never upgrade beyond that, or be in the unwanted situation of having to lug it around forever because it's situational. If you can effectively learn the stats off gear, and then put points into setting those stats without the gear, this isn't an issue though... It strikes me as a very good middle ground to strike, we'd get much more interesting and varied stats, but we wouldn't be locked into using Thief's Knife until the end of time, we'd just use Thief's Knife, learn Treasure Hunter off it, then continue upgrading gear as we do and setting Treasure Hunter in a cross-class skill style menu.
It's actually a suggestion partly brought up because what you've described is kind of already the case... No where near as bad as XI was given how quickly we turn over gear, but it's still there... If you have i230 gear already, what is the new dungeon gear doing for you exactly? Mhach gear might hold some value, but it also might now... I'm working on a relic, why do I care about the new weapon introduced in a trial? I don't... If they implemented interesting stats in a method similar to IX, then I'd perhaps care about that dungeon gear as gear, I'd want a Nidhogg weapon alongside my relic weapon, the stats would entice me to do that content much more, while not locking me down to using the same piece of equipment for the next two years...
Arguably that forces people to do all content, but... That's not how IX worked, you had more stats that you could possibly set... All SE needs to do is balance it so you get fairly useful and generic skills off common gear that pretty much everyone gets, while rarer gear has more situational or impressive effects... If you really don't want to do Nidhogg, still don't, you'd still have stats off your current gear to set, and I can picture Nidhogg rewarding something extremely situational (Dragon Killer).