Yup, pretty much everyone failed, including Costa. I mean he succeeded in getting someone sent off, just failed as a human being.
Thing is if Costa was given fairer treatment in general over the past year. If when he dived he actually got carded, if when he puts his hands in peoples faces he got red cards he would have entirely stopped this behaviour already.
Players cheat because they get away with it, no other reason. If you get a yellow/red card every time you do something the risk vs reward becomes unfavourable. Players dive because they get penalties more often than being sent off, the risk is worth the reward. If 99% of dives got a yellow and almost never a penalty the risk isn't worth the reward.
This is why I'm so pro video replay, if 95% of cheating is easily caught 99% of cheating will stop because there is no point in cheating in the first place. It's also why the argument that it will ruin football is ridiculous. We will literally have a month of hilarious games over 8 v 8, a quick learning curve and almost a complete stop in diving, shirt pulling in the box, bad tackles, hands in faces, weird fake fairly gay forehead massaging, etc. The game will be less interrupted that it is now by the cheating itself.
Either way if refs did their jobs better last year Costa wouldn't be behaving like he is now. Letting him get away with this crap(and Suarez before him) actively encourages this behaviour.
If Costa does get away with it not only will he do it again but other players will see how it helped his team and they will do it themselves.