If it was linear then lvl 18 item could very well be as good as level 21 one and IMO that's just not good there as it makes things stale.
That one legendary rings pretty much serve as a good example.
2 huntsman 1 warfare on a ring, I think it was level 12 or 14.
Even level 20 divine ring can't create an option as good as that. When I replaced the ring, it was simply because I finally needed more magic armor in a level 20 boss fight.
