So there was A LOT of hate when mythics were first announced. I was a naysayer myself. My biggest concern was that the new rank would just start the power creep issue all over again. However, it appears that Social Point has been a bit more careful with mythics. I keep seeing some common reactions whenever details of a new mythic are announced: "his skills suck" and "terrible trait."
That seems to be intentional on SP's part. These mythics aren't useful because of their skills necessarily, but because their stats are so high that they can live through what some of the best legendary monsters can dish out. They are purposefully keeping some of the best status effects (area dodge, mega taunt, etc.) out of the hands of the mythics. So long as they keep doing that, my negative concerns may never come to pass.
What will they do when they feel like they've run out of ideas and want to push the boundaries again? I don't know, maybe then my fears will be realized. Or maybe they'll introduce a new legendary monster to bring in some new potentially game breaking mechanic, rather than a mythic. We'll see, but I don't think things are as gloomy as I originally thought.