With regards to wanting a lower sync so the dungeons are more "challenging", I think it's important to keep in mind that the dungeons that sync to ilvl 110 are older dungeons that had been challenging at one point, but just aren't anymore.

I figure most players have run these into the ground, and making the sync lower wouldn't add any challenge or fun, it would just make the grind even more repetitive, where it actually plays out pretty smoothly now. The dungeons would have to be completely remade to get what you want, and at that point it's better to make new dungeons instead, which is what I think square enix realized.