Popular multiplayer games like this can have very strong staying power, especially if they become eSports. It's not a minor diversion like Flappy Bird.

It'll probably last a long time yet. Though anything could happen - a number of Battle Royale games have come and gone. But they've tended to live on shaky ground and make player hostile decisions, whereas Epic has a very solid engine/platform and they make better decisions.

It won’t be going anywhere anytime soon. They just recently announced $100,000,000 (not a typo) of prizes for the upcoming year. Lots of pros that snubbed their nose at it over the past few months are now doing half Fortnite streams ok top of their core game as they try to see if they can be competitive

Of course it's a fad. That doesn't mean it won't make billions of dollars.