Boots and Tenacity or Ninja Tabis and Merc Treads

I feel Merc treads are often bought simply for the tenacity rather than as an anti caster item. i have a couple ideas that could solve this. Any input rioter or otherwise is apreciated

FIrst i want to say that i see merc treads as an anti caster item more so than an anti AP item, similarly i see ninja tabis as an anti Auto attack item rather than anti AD. my ideas will strengthen that relationship.

now to get to the point. I feel tenacity does not belong on any one boot. it would make alot more sense of it were moved to a tier 3 enhancement. or removed from the game entirely and the mastery strengthened (yes i know that in te case of removal the other items that give tenacity would need compensation).

merc treads would also need compensation. there are many ways it could be applied but the ones i was thinking were , spell reduction same as ninja tabis is auto reduction, although perhaps a a lower percent, or it could simply be AOE reduction or dot reduction if something as broad as spell reduction is op.

i also feel ninja tabis could use a bit more impact that would undoubtedly raise its price. i think they should reduce all damage applied via auto attack. including on hits. this would allow you to build it against teams or champs that are AA reliant. teemo lulu poppy yorick nasus etc whether or not they are ad.