Here's what I have learned from MMOs...
There will always, and I mean always, be a group that flies through content and unsubs. 99% of the time, that group will come back once content is released.
Now, I do expect some of our raiders to unsub because of zero content. In fact, some of my friends already have which is extremely unfortunate.
I do think, and I'm only speaking for myself, the story players will stick around. I will remain subbed until the end of Season 1, at least. Of course, I have multiple characters and plan to take all of my girls through the story.