Conqueror gives so many free stats that it autowins fights. Who though giving free flat AD/AP, true damage, AND sustain was a good idea? and then make it a keystone that can be kept up for the entirety of a fight no matter how long it goes on as long as the user can keep adding stacks?
ADC's are taking it over fleet footwork because the healing is infinitely better and the free stats that it gives is too good to pass up.
Assassins like Akali and Zed are able to use it since they can stack it so quickly they essentially get free damage and true damage throughout a fight.
Bruisers are taking the keystone not only because it was built for them, but it outclasses aftershock, grasp, lethal tempo, and literally every other viable top lane keystone to a degree that the phrase "seal clubbing" comes to mind. Its absolutely absurd. Twitch, nasus, garen, and olaf should not be all able to take the exact same keystone. But they can, and are, and are winning with it.
So many of the champs taking it already have sustain and true damage in their kits. The added bonus makes this absolutely oppressive.
Patch after patch after patch this keystone hasn't been touched. It has absolutely no drawbacks.
Grasp takes just as long to wind up (even longer if the conq user has spammable abilities and auto-resets that can grant near full stacks in under 3 seconds) and only provides a single heal based on max HP (read: practically non-existant pre-15 minutes.) followed by a cooldown. Conq has no cooldown and runs infinitely once stacked.
The flat stats that aftershock provides does nothing against conqueror. Sure, the initial proc might negate the bonus AD/AP that conq gives, but the true damage is still going to proc for far more than what is negated. Not to mention the free sustain conq gives will absolutely heal any damage aftershock would do.
the attack speed that lethal temp gives comes nowhere near to bringing the damage output that conq does and at this point press the attack is just a shittier version of conqueror. It gives less bonus damage than conq does and doesn't' come with sustain and also only procs on a single target.
Why take klepto when the amount of gold you'll get wont come close to making up for the free stats that conq gives right away, no rng involved?
Glacial augment wont help you kite unless you're playing a marksman top, aftershock does less damage and will be immediately healed through, dark harvest takes too long to scale and wont match conq's damage until very late in the game, predator is too situational and also gives you far less damage with a very long cooldown.
So what's left? Support keystones? As if aery, comet, guardian, or unsealed spellbook are going to do anything for you.
_**TL;DR? Conqueror outshines every single other keystone in the game by a large margin and needs a nerf.**_
Take away the sustain, the true damage, or the flat AD/AP. Do SOMETHING.