Okay. I can live with that. There is a far better incentive after all. I don't suppose this incredible surprise will be enough to respect your decisions in the matter?
Anyway, it's flat-out awesome that Thieves finally became bona-fide Treasure Hunters, and that NOT EVERYBODY CAN DO IT! That was starting to really get on my nerves. I like the idea of heading down into the Dungeon and not having to worry about whether an item will be a rare or a common elixir, or if SOMEONE WILL SNATCH IT BEFORE I GET THERE. You can stuff duplicates of rare items down there for hardcore types who think they can juggle heavy fights while searching for treasure. How else can the Thief legitimately brag? Keeping them optional, by what you prescribe, should lessen the pressure on whoever just wants to brawl.
Anyway, if you're thinking about using a different creature other than the Byblos, or a special character, why not have the Byblos featured earlier, in some other battle?
Also, should you nerf rare finds, why not elevate common stuff a little bit by adding some features that make them unique or reprehensibly functional in some odd fashion? After you reach a certain level, the stores' stuff just doesn't cut it because they raise one stat without added functionality. For instance, there must be some method of altering armor to raise Health by a percentage instead of a lump sum. Then we can scale equipment differently and give ourselves an excuse to add special effects to the piece. But that's probably lightyears ahead of the curve, so nerfing rarities is okay for now.