I agree, either Tenacity should work on displacements (and probably just speed up lateral displacements to not create range discrepancies), or Tenacity shouldn't exist at all. For quite some time now, Riot's default answer to giving a champion a knockup has been because the effect is clearer and more consistent than a simple stun, implying that Tenacity takes away from that.
Many champions, especially those with large amounts of CC on a single ability like Fiddlesticks, Morgana, Rammus, etc., tend to have their strength affected by Tenacity, and are therefore balanced around it. By contrast, more recent designs like Rek'Sai and Yasuo were given knockups so as to avoid that extra bit of variance. Trouble is, because these mechanics come from two opposing mentalities ("we want an effect that counters CC to be able to deal with it better" versus "we want this CC to not have item-based counters so that it can be more reliable"), this leaves the players confused, and makes it look like either design is unfair. Morg's CC is unpleasant because it's a 3-second root, even though you're expected to get Merc Treads and reduce it to 2 seconds, and Yasuo's knockups don't feel great when you do have Tenacity because displacement just so happens to be one of the few special CC types that ignores the effect.
What we need is a level playing field, where either all CC can be countered by Tenacity (and CC shouldn't be made uncounterable for the sake of reliability), or Tenacity just gets phased out, and kits that were balanced against it get made easier to deal with. Personally, I'm in favor of the latter option, as I don't think counter-itemization in League really leads to good gameplay, but either one could have a significantly positive impact on the state of crowd control.