This would assume blizzard would spend the entire time making a legion wrath or TBC kind of expansion, and not half shit, half decent, so we'd basically just end up with a semi decent expansion.. of which case why not just leave one expansion to one story?
Hey i'm sure we'd be fine if they said "Okay shadowlands storyline is going to take an extra year, but we are going to keep making content."
However, Blizzard or more Activision doesn't want that.. they want the money every X often, the sub isn't enough they also want to sell a $60 expansion as often as they can.
and if they charged 120$ and fucked it up majorly they'd be set for low subs for so long before they could bring empty promises with the next expansion fixing all the issues.