AOS was always touted as a narrative that would progress as time went on, so "the current event is only being done to introduce the next event" will always be true.
The Realmgate Wars existed to set up the Season of Hope, which was there to set up Malign Portents, which will probably lead to some conflict with Slaneesh, then that will lead in to something to do with the ogors or goblins, and so on...
It's a marketing tactic to keep people actively engaged in the product line as a whole rather than getting tunnel vision over their specific faction getting updated maybe once every 5 years.