Was SBMM a mistake?
I will admit that I'm a bit biased here. I personally think that Dbd is meant to be a casual game, and that it should have stayed that way and that the addition of MMR kind of destroys anyone trying to play less competitively. You could argue that you could afk or purposely derank in some other way, but someone shouldn't need to be forced to waste their time in such a way in order to enjoy playing. I'm sure a lot of people out there will agree with me. I don't like going against the same killers, builds, items, strategies, perks, addons, etc. over and over again, and I don't like that I'm just left in the dust if I do not do the same.
On the other hand, I see why a lot of people support the MMR system. Matchmaking is more consistent than rank based matchmaking (which isn't saying much) and matches generally feel more competetive, and i understand why many people like the tension or adrenaline that comes with that.