A strange new opposing force will almost always seem like the very thing that'll dominate the old and familiar. It happens all of the time. If people want to move on to something new, why should it be a concern? It's impossible for something to appeal to a mass of people for an indefinite amount of time. Eventually people will move on to what they consider to be better for them, and that's perfectly fine.
I'm inclined to believe that Fortnite will be potentially strong for its own reason, but I don't think it's the type of game that will overtake a MOBA giant. The gameplay is just too different, and appeals to a different style of competitive gaming. Given that the game is not fully released yet, I feel like it's unfair for me to give a solid opinion, but I'm doubtful.