Boots to counter CC chains specifically.
Mercury treads could maintain their value for lets say that Fiddles Fear, Soraka Silence, or Ahri Charm... that one ability on the enemy team that can ruin your game.
A pair of boots where they dont really reduce the first CC as much as mercs or but will be slightly more effective when hit by another in quick succession. If hit by a third it becomes tremendously more effective stacking up more and more with every CC applied to you to a reasonable cap.
1st CC you would benefit from a base tenacity of about 15-20%. If the second CC hits during or within 1-2 seconds whithin the first CC then it could scale up to 35-40% tenacity, then with the third giving say about 55-60% tenacity all the way up to lets say about 80% or so. However after the 1-2 seconds following the last CCs duation you lose all stacks. Point being a trade off to mercs where if you think you might be perma CC'd you get more of a window where mercs is better in situations where the CC is going to be in increments.