You seem to be forgetting, that most of those champs existed during the time of the Meta Golem. And guess what, it didn't matter. The metagolem warped the game so much that it was either, you pick 2-3 champs that can build metagolem effectively and have the ADC/Tanky Support/AP mid, and suddenly you've got 3 champs all with 3k-5k HP, one with hard engage, 2 with hard followup, and a crap ton of super damage.
Yes, BotRK and Liandry's are a thing, and there are a ton of %HP damage in the game, but many of those items or some form of %hp damage existed in League when Metagolem was around. The problem was, the original Metagolem had Wriggle's Lantern, which gave 15% lifesteal, So yes, the return of Atma's would be "Weaker" but the problem is, with Cinderhulk multiplying the HP gained from the Metagolem build (which gives around 2000 bonus HP without cinderhulk) The minimal resistances in the build are still incredibly effective.
Also, many of the Metagolem champs had things that made their HP more effective.
Has his Oranges for heal and CC cleanse. His Q procs Sheen, Haste, Frozen Mallet Slow and his passive, AND can Crit. He Q's you once, then charges at you and you can't escape.
Has his Q for gap closing, E for slow, and W for insane heals.
has a parry, and her Ult, each strike of which applies on hit effects, so the first hit will use and consume sheen, each hit will apply Frozen Mallet slow, and TF haste. Once she's done, if you're still alive, she will be unstoppable in terms of getting away.
Has his W% max HP, a leap, and a Slow. Once he hits mega form, you can't get away from him. He'll chain CC you, weave in autos that proc sheen, and slow you even more. Good luck killing a 5k hp mega gnar that has 3.5 seconds of AoE stun.
His Ult and E mean that he doesn't have a hard time getting to you. The haste from TF gives him more damage, his Q allows constant usage of sheen, and his W gives him HP from damage.
She has dash and a 2 second targeted STUN. She does true damage on hit, is already a huge user of Trinity force already, and add in Frozen Mallet and you have no chance of escaping her when you already had so little. If you DO manage to get her Low, she can stun you. Her ult heal scales off of AD, and her W grants HP on hit.
Has a shield, a knockup/armorshred and a gigantic gapcloser that does a sizable amount of damage. He was THE abuser of Metagolem build back in the day. Give him the tools to become that monster again, and he'll become the forefront of the bruiser force.
Again, another champ that abuses Trinity force already. He has gigantic resistance Steroids that are going to be getting buffed next patch, and with AoE stun and a leap and his Dodge. You can't really stop him without heavy focus right now, with Metagolem, he's even harder to deal with.
Another champ that loves loads of bonus AD, Lee Sin was a heavy abuser of trinity force back in the days before hydra. If metagolem comes back, his insane AD scalings mean that he would love to build tanky and just become an unstoppable monster. With his Shield and spellvamp/lifesteal from his W, he's already a pretty insane duelist, and metagolem pushes him over the top.
With Nocturnes Ult and spell shield and his Q and Fear, Nocturne loves things that make him tanky, and make it impossible for his targets to run from him. Frozen Mallet+trinity force gives Nocturne all the damage and utility he needs to stick to targets. His passive HP gain on attack makes him slightly beefier.
Shyvanna doesn't have need of many damage items because of how much freaking damage is given to her for free on her Kit. With her high natural resistances from passive+dragon form, Trinity Force and Frozen mallet help her issues with kitability because once she REACHES a target, they can't escape, and with her Ult, she can reach someone and then be unable to be stopped.
Skarner is champ that loves being able to stick to someone, and since he loves bonus AD along with HP, he does very well with metagolem. If he flashes to ult someone, they won't be able to get away, even if they flash after the ult is over, because the slow from Frozen Mallet plus the haste from Trinity Force makes him as sticky as glue. His Shield also makes him even tankier.
The pure epitome of fighters and bruisers. With buffs coming next patch to his ultimate's Resistance steal, Trundle doesn't need resistances to become a nightmare tank. With Q activating Sheen and consuming Sheen and procing Frozen Mallet, Trundle becomes an unkillable nightmare late game with the Metagolem.
If he ever manages to reach you in bear stance, you won't be able to escape. Udyr's very mechanics and gameplay style lead to incredible synergy with Trinity Force already. Give him massive bonus AD from Atma's, and his Turtle stance lifesteal and his Tiger stance DoT will tear everybody to shreds.
Vi also loves trinity force, and with an unstoppable ult with bonus AD scaling, Vi would probably become one of the most overpowered champs in league with the return of Atma's. Her Shield, %Armor Shred and dash mean that she can catch any fleeing target, and once she reaches them, they may as well just sit down and weep.
Garen synergizes well with the Metagolem because of his passive and E scaling with Crit chance. He's more vulnerable to kiting, but if he does manage to reach you, Trinity FOrce haste and Frozen Mallet slow mean that you can't get away.
The last of the bruisers. his point and click dash/slow means that you won't get away even if you expected to already. with Frozen Mallet and Trinity Force, trying to kite Xin will be impossible after he first reaches you. Plus the fact that he gains HP every 3rd auto, means that he's fairly tanky without building much resistances.
Overall, every bruiser and fighter can take advantage of and ABUSE the living hell out of Atma's.
If you try to change it's passive to something based off of current HP, then it doesn't become worth picking up or is so worth picking up that it ends up getting nerfed and thus never picked up.