So I was playing a ranked game and randomly two teammates decide they have had enough and leave the game entirely even though we were in a really good spot. Anyway, the game goes on for another 15 minutes and ofcourse, the enemy team stomps us 5v3. How in the hell is it fair to lose MMR that game? Okay sure, let the enemy team gain MMR normally, but why in the hell should my rating suffer because of two knuckleheads that made the game literally impossible to win.
It’s more about scale of the imbalance, since as you notedd there are already factors like 4v5 teams winning or players being capped to 1 MMR on the lower end that impact the economics of MMR.
The big differences are that most of those scenarios only have like 1/10th the impact (for example 1 player vs 4 then figure like a 60/40 uphill win) a flat “you have a leaver you don’t lose MMR” policy would, and that they aren’t easily abusable in the same fashion.