With so many flex options it is close to impossible to tell where people are going, and therefore hard to get the right defense stats picked. Or just nerf mage AP so they do not have infinite mana and access to burst.
Tormentula2 votesMost AP have no reason to get AD, so it adapts to AP.
Most AD have no reason to get AP, so it adapts to AD.
Most tanks benefit from both armor and MR since they're resistant stats, even a full AD team could have some mixed magic damage tied to their kits, or tanks could just nullify some damage from auto attacks. Adapting it makes x or y less appealing. Everyone benefits from having either of those stats, even if you're not primarily building into them.
The "adaptive" tank stats simply put, are choosing between MR/Armor. What if a mage wants to take MR vs. another mage, but they build seeker's because rengar is camping their candy ass and become more vulnerable to opposing poke because it adapted to armor, they can't get best of both worlds when with old runes, they could. (and nothing with old runes really ran mixed damage, but yellows and glyphs were commonly always armor + MR or health + MR)
Power Cosmic1 votesWe can all agree that lane matchups are the first priority of any game. Mages are destroying top simply bc of the game's raw numbers. A dual damage rune mage that goes top when they already had a Jax and Panth is going to punish you severely for being normal and taking armor. Trading lanes is cool but hardly anyone is willing to in ranked or normal.
iiGazeii1 votesAdaptive tank is called "health".
Power Cosmic2 votesAdaptive tank is called "health".
Don't be that guy. Health is called health. There is no Adaptive tank like there is adaptive damage. Armor and MR make health go a lot further.
iiGazeii1 votesIt changes based on what they've built, not what you've built. If they have AP, it does magic damage. If they have AD, it does physical damage. If somebody is building AP, build MR, and it will reduce their adaptive damage. Same goes for AD and armor. There is no guessing involved.
Power Cosmic1 votesI, and everyone else, knows how Adaptive damage works. Adaptive tank, however, we could theorize how to implement that.
Tahminatrix 1 votesBasically what op wants is similar to a generic damage reduction like on alistar ult or 
Since damage reduction that isn't based on armor or magic resistance doesn't care if you do magic dmg or physical damage.
Atleast that's the best way to implement this on an item or rune, it would be complicated to add armor that counts as magic resist and vice versa.
Yes I am aware galio`s generic damage reduction is "based" or "scales with" his mr, don't be that guy. Don't do it...STop….STOOPP DONT POINT IT OUT...NOOO.