See, the only reason it would demote you is if your MMR is pretty darn low, so odds are all your games "is S5" were probably against B2-B3 players. It's also generally a bad habit to blame a loss on a feeding team/enemy smurfs (fyi most smurfs win their placement matches and come out around Silver-Gold so the amount of smurfs in B is comparatively low, there are people who are climbing but that's different), whether they were the reason for you loss or not, it's better to focus on what you could have done better and prepare for the next.
Also I understand the frustration attached to that because I had to work my way out of B1 twice. Although I was back out the second time really quickly, focus the frustrated energy towards a positive direction.
Good luck!