There seems to be a common perception that once Galaxy's Edge opens there would be a mad rush of guests.
I don't buy that. Personally, my vacation plans are generally set towards the end of the year. I need to confirm vacation availability, and if I were planning a trip for GE specifically, I'd need to know the opening dates. By the time Disney announced the (partial) opening, my 2019 travel plans were pretty much set.
Many will still wait until the signature attraction is ready.
There may be a bit of an uptick in guests who want to visit in September, but I don't see demand being so strong that discounts will go out the window. There's reasons Disney consistently offers discounts for this period, including but not limited to many families not wanting their kids to miss the first week(s) of school.
I don't see GE as something that will draw guests RIGHT NOW! It will have an impact for years to come.