DMs don't have to do any of that right now. I have yet to encounter a single DM who wasn't interested in running AL adventures because they didn't know the plotline of a previous season's adventures. Most people don't seem to care at all.
What prevents them from being reused now? Nothing says we can't have an adventure in season 6 that gives out a headband of intellect just because there was one in season 1. Similarly, retiring a season because of a problematic item doesn't get that item out of circulation. Plenty of people have already played that adventure. If something is bad enough to require removing it then it requires campaign level errata, in which case you can just errata the adventure.
And honestly, if you're worried about power creep your target should be the hardcovers, not the previous seasons of AL content. That's where the really broken magic items are.
Not only can they reuse treasures from previous adventures without retiring those adventures, they are doing so. The one example that I will use is the Bracers of Archery which were in a late season 3 adventure and an early season 5 adventure.
If the rationalization for retiring adventures is the power level of the items, then it would definitely be the hardcovers that should be looked at.