Thank you for your insight -quick question - why do you say next year will be much more unpredictable? Is there something new I am not aware of presently?
Up until now, Disney Vacation Development has released rooms to stay ahead of points sold. What that means is that there have always been more rooms available than there have been Poly owners. Some excess capacity was going to be available at 7 months.
Once sold out, there'll be a Poly owner for 98% of Poly booking nights. Since most of those owners didn't buy enough points for bungalows, on any given booking night, there'll be more Poly owners with only enough points for studios than there'll be studios.
Some will book elsewhere at 7 months. Some will use points for cruises, etc. More will book during prime time than off DVC season, skewing demand.
How many? Who knows? Until we see it first hand, there's no pattern from which to make an accurate prediction.
Unfortunately for Sept 2018 bookings, that will be the first time booking Sept as a fully owned resort for Poly. Instead of following a previous example, it'll be setting it.
Plus. Early Sept has traditionally been DVC slow season and busy season didn't start until F&W in mid September. But now WDW has moved F&W back to the first of Sept.
The combination of the events, more owners than studios in a new busy season will create uncertainty about booking patterns until we see them in action. Both of those things will be new and not factored in before. So, next year will have a very different booking pattern for September than this year.
If you're planning to book Poly studios in Sept, 2018, then Sept 2017 is not a reasonable guide for what to expect.