Critical Ring is replaced by what?

If you are a hero it is replaced much more easily.

If you have under 171 total attack, a 5atk ring can replace it.
If you have under 229 total attack, a 6atk ring can replace it.
If you have under 286 total attack, a 7atk ring can replace it.

The Angelic Blessing / Dark Angelic Blessing rings and Evo rings are the only ones able to do this anyway.

I guess the point is, 10% crit is not easily replaceable on any class, and for most the critical ring is the best of the 4. The main downside is that they're untradeable, so you need to train up a bunch of mules 40-80, then delete them.

For the same type of comparison, other classes: you need to have less than 86 total w.atk on any other class in order for a 5atk ring to perform as well as the crit ring, less than 200 w.atk for a 10 atk ring to perform as well as the crit ring.