Given that FP- was always a "day of" thing, I suspect that many guests don't know about the pre-booking options, or simply don't think that they need to book in advance. Perhaps that will continue well into the future, but I suspect that through marketing and occasional disappointments, ("I couldn't get TSM or E&A") people will learn that earlier is better. As early booking becomes more and more common, more and more supply will have to be shifted in that direction. There is an obvious and unavoidable tension between moving slots over to the "pre-booking" side versus holding slots back for the "change on the fly" side. I'm guessing that most people, (and folks here can tell us what they think), would prefer more options when they pre-book than the ability to change on the fly. In other words, given the choices of:
A. More options when pre-booking but fewer opportunities to switch once in the park; or
B. Fewer options when pre-booking but more options once you arrive at the park and more opportunities to change once you are there
most people would choose "A". Ideally, people will want both, but that is a tough thing to pull off. Personally, if I am dropping $5K on a vacation where pre-booking attractions is the latest and greatest thing, I would rather leave home knowing that I have my FP+s for the headliners in my back pocket than board the plane knowing that I have nothing, but flexibility will likely save the day for me. I'd hate to arrive at Epcot at RD, then head straight to a kiosk at 9:00, (or log on with my phone) and find out that, yes, certain slots were held back, but the best I could get was 6:00 p.m. Had I known that in advance, I likely would have tried a different park that day.