Before Fortnite was Minecraft and Pokemon Go and Angry Birds... It's in the nature of video games to come and go like fads.
More telling (to me) is that Epic started making a game very different from Fortnite but was both flexible enough and perceptive enough to change the game.
The difference is that Epic is leveraging their current popularity to turn into a platform. Pokemon Go and Angry Birds were already only really available through platforms, so that wasn't an option for them. Minecraft maybe could have, but it seems somewhat opposed to the idea of the program, which is all about user control and users creating an ecosystem. Now that Microsoft owns it, there's already a platform for it, and it's not game specific (the Windows Store).
Fortnite has as many or more monthly players as Minecraft (which is huge), but much more control over the ecosystem and content. As the controlling party of what's (by this point most likely) the most popular online game in history in it's peak (until next month, most likely), that gives them a somewhat unique and powerful position, which they seem eager to exploit.
Will that be enough? I don't know. I definitely think the situation is different than the examples you cited though.
More telling (to me) is that Epic started making a game very different from Fortnite but was both flexible enough and perceptive enough to change the game.