You really have no idea how broken this Zephyr idea is.
Udyr win rate will spike to 59% Almost overnight.
Xin Zhao will spike to 56%
Nocturne will spike to 57%
Tenacity stacking is NEVER a good thing in competitive play.
This will bury mages, simply by the virtue that every game will = 72% Tenacity, 70 resist from the same item, and no real offensive downsides on many champs that would use it.
There's also a reason Riot stopped having 10% movement speed items in large numbers.
Zephyr was pre Dead Man's Plate/Righteous Glory/Out of combat Youmuu's.
If Implemented, Fighters will be faster than every other champion in the game in almost every situation, be un CC-able (except knockups), be mage-proof, and have attack speed built in on the same item.
The result will either be massive scaling AP adjustments to mages to bring back AP into items like Rod of Ages, to create more 120+ AP options, possibly a Void staff buff, and individual kit buffs on mages who already are hard to deal with (Some champions won't be rushing this item, like adcs, so all these compensation buffs would result in them dying faster to champs they already often die from, like Ahri)