Can we lower tank's damage...

And while we are at it lower the damage for all other classes too?

A good 15% reduction or more of all damage would probably be appropriate. Lets take a look at a picture and some math for reference. https://imgur.com/6FYthfW

K, so this says that a kassain killed a lux in .79 seconds with 3085 damage.

I was on that smurf account against a smurf kassadin... That actually happened. I died in that amount of time. Now lets go over some numbers. I had 38MR which Sure isn't much, but that is 27.5% damage reduction... He did however have 18+8 MP so that means that I had 12MR for 10.7% damage reduction which means that he had to deal 3454 damage in under 1 second...

If you were to confirm his damage with the lichbane and ludens the damage screen is correct. Its actually missing about 100-200 damage, but we won't go over this.

Now lets go over what is wrong with this damage. He over killed me by 600 damage, and didn't even quite hit me with all of his damage. Now how are we supposed to play against this type of damage? Believe it or not, there was a time where being full hp as any champion meant that you wouldn't be instantly nuked. It would take at least a few poke spells to get a champion to half hp and then you could finish fully comboing them. It allowed safety when you were full hp, and allowed a semblance of a lane phase farming minions rather than creating a snowball meta. One of the biggest atrocities to this that it isn't just assassins overkiling their targets its all champions. ADC's, Tanks, Damage Supports, Mages and the likes.

If we lower damage we allow weaker laners to farm into the game unless the enemy is really good and gets more than one rotation off. It stops tanks from just abusing the enemy with higher upfront damage. It allows healing champs like aatrox, vlad, morde,soraka to show more of their sustained fighting prowess, and it even forces adc's to use their kiting skills to fight rather than just standing still and acting like a turret. It even allows adc's to possibly survive a diver getting to them without having to blow up the diver instantly.

Lowering damage increases counter play, not reduces it. Insta-killing is the epitome of 0 counter play .