Nerf what is strong so weaker champions don't get buffs.

It's hard in practice to always do this. A lot of champions are only strong because of the Meta they find themselves in and vice versa. Assassins during a tank meta don't need buffs because as soon as the tank meta ends they'll be broken op. It's sometimes hard to say what exactly is making a champion overpowered. For Master Yi it wasn't Yi himself that made his damage retarded it was Guinsoo's Rageblade being a broken item in general that in turn broke on hit champions that could abuse it really well. Nerfing him would have been a large mistake at that time.

Typically if you feel like you've got the end all solution to a problem that has existed for a long time you're either much smarter than everyone else who has ever thought about the problem, or you're missing something. If you believe the former to be true then you're arrogant.