an old interview on the MMR system
Let's suppose what you say is true - this would be the case for casual mode since conceding in legend lowers your MMR. The MMR in the new ladder system is lowered too after a concede since many people report they face easier opponents that way. And since Blizzard haven't stated otherwise explicitly in their FAQ (where they commented on MMR tanking from Season to Season) this pretty much means your MMR drops when you concede.
Tried to ladder a bit today and was closer than ever to deleting the game. This is complete and utter shit.
Just give it a break for some days and you'll be fine.