Tenacity rewards bad players who don't know how to position correctly.

You’re very wrong. If League was a game where a full team comp had maybe 3 CC’s on a decent cooldown, I could understand. League is a game where basically every champion on the team has a slow + 1 hard CC on short cooldowns. Certain champions need tenacity to be relevant.

also don't be a bitch about people using merc treads when you built them 2 games in a row on vi

Should be more tenacity so cc doesn't last twenty decades, some of us have to be the engage here

Tenacity is fine. Even with it, it does not prevent players from landing full combos.

If tenacity was on items which were available for squishy high mobility champs to build I would agree, but tenacity is on bruiser and tank items usually. They have less mobility and larger models so they're easier to get hit by CC. It feels very unsatisfying to play a tank against a hard CC comp already, tenacity is there to alleviate that.