I was active on neopets when, after years of constantly having 80-100K players concurrently online they fell to 2-8K players concurrently online. The only new players that came into the community were people who got frozen for cheating and were pretending to be new.
League hasn't had the 90% playerbase loss that neopets has had. And even if it did, that would still be several million users monthly.
League isn't anything remotely close to dying.
It's just that Fortinite is a decent game and is carving its own place in the scene. Sure lots of people will start playing Fortinite, but league isn't going to lose that much of its playerbase. The games are different.
PUBG will die though.