Every class, and every champion is effected differently from a flat 10% damage reduction across the board.
As many posts on the boards say "Riot should just reduce the damage across the board by 10%". This simply won't work, and the reason why should be fairly obvious.
If a champion X does 1000 damage, and a champion Y does 1100 damage, and a target has 980 HP, pre nerf both champions will one shot the adc with one rotation. A 10% nerf would GUT champion 1, since champion X now does 900 damage, and must wait for their next rotation, while champion Y still does 990 damage and can one shot the target with only one rotation.
This becomes even more complicated when cooldowns and skillshots are taken into account. classes with DPS will not be as badly nerfed as classes reliant on burst. To conclude, a 10% nerf across the board will cause more problems than it will actually fix.
Yes, damage is too high; but a generalist approach to toning down damage is not the best idea. A more specific approach with champion specific/keystone/gold generation nerfs are better.