So when do you actually get demoted?

This guy from my last game is on a 8 game losing spree of hard feeding with a 47% win rate in plat 5 at 0lp for a while now. 28% win rate in last 20 games..

Clearly the demotion system needs some work.. In melee terms it would be like going 0-2 every single tourney and still getting placed at like 10th seed when you should and have shown to be like 20th.