I hear this argument often, but there are counterexamples like Minecraft, CSGO, World of Warcraft (one of the few MMOs that still have a subscription).
So it's entirely possible that Fortnite will stay.
Minecraft has arguably evolved into its own "platform" with people building and sharing their own worlds (some on Minecraft's own managed Realms, many not) and tons of third-party games built on top of Minecraft accessible to the public. Fortnite may evolve in this direction some day, but it hasn't yet.
It's all about updates. If there is new stuff to do, people will come back eventually. Maybe not every year but there are some games I've come back to every 2 years.
So it's entirely possible that Fortnite will stay.