I don't think smartphone gaming is a fad, in fact, I firmly believe it has already made it's place in the market, and is going to stay there. Smartphone gaming, and let's face it, free-to-play type games altogether make a lot of money. I mean, A LOT of money. With games like FarmVille making over a billion dollars a year, and other like Clash of Clans quickly approaching that margin, if not already breaking it.
Just look at Angry Birds for example, while it is the most successful of the games that we've seen come out of smartphone gaming, it can honestly be compared to really any other major franchise like, Fruit Ninja, Temple Run, or even Cut The Rope. These games aren't just getting games; they're getting toys, clothing lines, TV shows, hell, even full feature films.
Another large example of just how successful smartphone games can be is in Flappy Bird. That game came out, and within a few days it was making over $50,000 every, single, day. All of this based purely off of word of mouth alone. Now, imagine if a game as simple and as effective as Flappy Bird had a whole marketing team behind it. Holy ****.
Successes like this are due to the majority of smartphone game's mass appeal. Meaning, they're all very easy to pick up, learn, and play. They usually are very cheap to make, hence why they rarely cost more than either free, with micro transactions thrown in for good measure, or upwards of 99 cents; allowing as many people to try it, and pay more for it as possible.
In short, smartphone games are too accessible, make too much money, and are to easy to make to be just a fad. Who knows, that could lead to an economic bubble of sorts, but I doubt we'll see that for a long time. But, whether or not you like it, smartphone games are here to stay.