Tank Yi is clearly not dead but I have an elegant solution.

Certain champions just need to be punished for building certain stat lines its the only way to keep them honest. We all have seen what happens when champions who are designed to be squishy damage dealers can build tanky and be successful. Tank ekko, Tank Fizz, Tank Yi, Tank fiora, Tank graves, the list goes on so instead of nerfing the items or "rebalancing" the champions why not make a simple clearer nerf that leaves the damage builds completely intact but severely reduces damage from tank builds. Malphite Rammus and several other tanks get damage scaling from building tank which is fine it encourages the most healthy build path so for non tanks who start developing these unhealthy builds give them negative scalings for defensive stats its easy simple and effective if yi builds health he loses 1% of his true damage scaling. If ekko builds more armor than a zhonas his ult takes a damage hit. Keep assassins, assassins and keep tanks, tanks and never shall the two mix.

Edit: (this is a satire on balance posts, Christ I thought it was more obvious)