I don't think Dragon-types lost that much of their viability. Don't know if I'm correct, because I haven't played competitive since the arrival of 6th generation, but from what I gather I can say that Dragon-types are not just a free ticket to win, but actually have a healthy risk vs reward balance, just like any other Pokemon type. Before, every team sort of needed a Dragon-type, and even DragMag/Dragonspam was a very viable strategy, with Dragon-types being disproportionately powerful in comparison to the other types. Seeing battlers put two Steel-type Pokemon onto their teams was not an uncommon sight, and maybe even required to have a chance of winning. A similar thing can be said about Fighting-types. In 5th Generation, Fighting-types were occasionally compared to what Psychic-types were in 1st Generation. No risk, big reward, and with the amount of heavy hitting Fighting-types introduced into 5th Generation, the introduction of the Fairy-type in 6th Generation was actually quite a blessing. And let's be honest, nobody is going to run Poison Jab or Iron Tail. This just means that there's a healthy risk to be considered when using a Dragon or Fighting-type, and its not just spamming a certain move or match-up related play anymore. This makes the metagame a lot more balanced than it was before, (and overall more enjoyable too, or so I've heard) :]
Anyway, if this happens to be incorrect, feel free to correct me, because as I said, I haven't actually played 6th Generation competitive and I don't lurk on Smogon quite as often as I used to either. This was more logical thinking and making my own conclusions :b