What happens when you lose with S + grade

I was just in promo to diamond 3. And I got this S+ rating from the rating system, but I still lost! I did very well, and I had even more gold than all of the players including enemy players. The way I see it is Lucian and Viktor in this game were bad and they were boosted by ranked system. It was not like they were not trying, it was that they were really bad players. I helped out my mid, junglar, and our junglar was good. But our mid and adc were really bad and I have to conclude that they were really boosted.

I played the ryze in the link. So my question is, what happens to your MMR when you receive S+ and still loses the game. Clearly getting S+ and losing the game tells about you and your team. I feel like if you get S + in ranked, you shouldn't lose lp or lose promo. So what really happens behind the scene when something like this happens someone gets S + but still loses the game?

Go to command prompt and copy this to see the replay: powershell clear;if(Get-Process "LolClient" -ErrorAction SilentlyContinue){$ErrorActionPreference="Stop";$c=New-Object Net.Sockets.TcpClient;$c.Connect("127.0.0.1",8393);$c.GetStream().write((16,0,0,0,1,0,0,0,0,0,0,0,69,0,0,0,114,101,112,108,97,121,32,114,101,112,108,97,121,46,103,103,58,56,48,56,48,32,54,83,78,97,121,98,53,121,104,52,109,75,49,66,103,90,66,99,65,117,85,115,119,77,120,116,90,115,102,78,116,48,32,50,48,50,48,52,55,50,52,55,53,32,78,65,49),0,85);Exit;}"Error. Please make sure your LoL client is running.";