Why would I even bother going merc treads when SO MANY champions have knockups and displacements that cannot be reduced by tenacity. Your ONE item that's designed to reduce CC you receive to help melee champions from getting stunlocked is USELESS against these champions:
And more.
Milla Jovovich1 votesYasuo listed twice,just to show that deep hatred :D
Btw Tenacity is very good against Thresh
Theorchero1 votesYasuo listed twice,just to show that deep hatred :D
Btw Tenacity is very good against Thresh
True but it's just an example of how pervasive displacement is. New champions get them all the time - and you can't reduce the cc.
Milla Jovovich1 votesYea new champs are usually overloaded with effects and stuff.There is no reason to play anyone before Xayah and Rakan in bot lane right now.
oSEXYPLATYPUSo1 voteswell you can get 
The Lost Marbles1 votesThe number of AD/physical champions on that list makes me question the logic of having tenacity on the MR boots in this day and age.
Can't we just move the tenacity to Swifties and have Mercs give AoE magic damage reduction (like one of the old masteries used to give)? Especially if we're trying to make AoE magic wombo combo a thing again.
That aside, yeah, tenacity probably could affect knockups with some finagling.
oSEXYPLATYPUSo1 votesthats not a bad idea but i like having the mr and tenacity together in one boot
Dark Nephthys1 votesWell tbh some of those champion's knock ups are generally really short to where getting it reduced would basically make it not even CC anymore. But I do agree to your point though, and knock ups/displacements are really annoying that is appearing more often now. I don't really think that Riot should make tenacity effect them but maybe create a new item that has it on it (either as a passive or as a active). Reasons why I feel it should be it's own separate item instead of is to make it as a down side to having it, make people have to spend the gold to get that resistance for it.
Just adding it on to Merc would kind of kill some of those champions since they're entire kit basically relies on them being able to knock enemies into the air, and we all know that if one pair of boots gets to strong that literally every role would abuse it (ex: 
).
And one of my examples of "champions that kind of rely on using their knock up would be ." If everyone basically had free knock up reduction, then she would basically be extremely weak which is saying something because she already feels kind of weak atm as it is)
Joxcab1 votesYeah we need Tenacity for a 0.25-0.5s knockup, wow, Riot so stupid wtf no Tenacity BSSSSSS