CoBeTHuK Diablo 4 failed because features of Diablo 3 that were expected (despite never being promised as included) were absent. Still, even the top games do this.
Notice the overwatch line though. Nearly the whole playerbase comes back when new twitch drops or events start. The game will definitely not "die" while people wait for content. They will return for the content, as you confirmed you can see, yes? All games do this, and all games have that wave to them. Never quite reaching their original peak, but not closing up in 2-3 months. This is normal. Surely you can see this.