When you warn with ample time, repeatedly, end up with 5 enemy on your side, and nobody comes because the rest of your team is all busy killing the last guy and you die and they plant, yea.... **** that ****.
You ask for a quitter debuff and what do we have now? We have people actually letting enemies jump on them to score. Or feeding kills. Or going afk at turrets. Or "moron stop capping so we can lose fast" So yeah, go live in your fantasy world where quitter debuffs fixes people. I know how it goes.