That's not how development at GW works. The Gathering Storm was obviously intended as a lead up to 8th Edition (with a big jump in rules), the Primaris Space Marine line, and the return of a fucking Primarch. Models are said to generally be planned at something like five years in advance, and I certainly imagine that being the case for a project as big as Primaris Space Marines.
Ynnari was just a small part of the campaign, an experiment to see if people would be more interested in the various Eldar factions if they could be mixed. Time passed without the audience biting, so the Ynnari were then sidelined as a failure, hence Gav Thorpe's incomplete Ynnari novel trilogy.
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Now the onset of the Great Rift is kind of sort of an End Times scenario since it clearly looks like a big step for Chaos, but it's really more like a shakeup to a new status quo sandbox setting since the narrative of the Imperium being in decline and besieged from all sides is certainly not new.