The "fallacies" of decreasing damage by 10%:

1.) Tanks won't be able to last-hit minions. Minons can have 10% less health and 10% more damage resistance to eachother to compensate. Boom.

2.) Champions will still continue to one-shot eachother. 10% damage resistance will not be enough to save someone that doesn't buy defense from someone else 2+ levels higher that is building pure damage; however in any other situation champions will lose their power to nuke people on their own, especially if any healing is involved.

3.) Games will be longer. Not if objectives are no harder to take down, if for example towers/monsters had 10% less health and 10% more minion resistance - again, all that needs to be reduced for there to be more varied gameplay is champion-to-champion damage, nothing else.

4.) Everyone will still run the same dumb rune pages. Objectively untrue - many, many runes will be infinitely more valuable than they once were if champions can avoid getting nuked by 1-2 champions and stats can reliably matter past, say, the 20-minute mark.

5.) Assassins as a role will die. Not die, but they will require more teamwork (gasp) to do well. Imagine 2 coordinated assassins/fighters/mages ganking being the norm - doesn't that sound cool? Cooler than one guy desperately trying to nuke people on their own at least?

6.) LC$ hypercarries and Big Play$ will cease to exist. Faker didn't first become famous in Season 8. League of Legends as a whole didn't first become famous in Season 8. Damage isn't what people get excited about. Search your feelings, you know it to be true...

7.) Riven skins won't sell. I'm sure they will. In fact I'm sure Riot will buff her to compensate no matter the circumstance, no qualms here.