Wouldn’t the 8/9th (dependent on time zone) December make sense for a new release date? The new offer for the 6 month sub ends on the 13th December. They would be able to capitalize on players returning for shadowlands, locking them into playing the first 6 months of the expansion, maybe not leaving time for a lapse in subs due to 9.1 being out or just around the corner.
This also leaves 4 weeks for raid release on the 6th January which is a good date.