I think you're greatly underestimating the percentage of players that like to use rings as glamour items. You're essentially saying that the amount of people who care about rings is far, far below the amount of players that have cleared UCoB ("less than probably 1%" vs 4.5%.).
When estimating the amount of people who care about rings, keep in mind that a small change to the 2B leggings caused enough of a backlash for Yoshi-P to personally address it during a live letter, and ensuring players that the leggings would be fixed asap. I'd imagine the removal of a ring slot would be met with a similar reaction from the playerbase.