Um, no they couldn't.
Remember, FP Legacy is a static system- a number of FP's are allocated per day at the distribution point and return times are incremented by based on rate of pull. There's nothing in place to allow them to dynamically manage anything- whether it's the ability for in-Park guests to make a reservation, time changes to reserved guests based on outages, etc.
FP+, being integrated with
MDE, is Disney's main driver for Park Capacity, Yield, and Throughput Management. Starting 60 days out, they now have the ability to know your Park of choice on each given day, what section of the Park you will be in at a given time, can influence your Park selection by making FP+ "suggestions", etc. And, they will be able to also do this from time of reservation through your full stay.
I HIGHLY suggest you read through the great work Bcrook has compiled regarding FP+ and the specific number crunching he and others have compiled. I think it will give you a much better understanding of FP+ and the huge roll it is going to play in all of this.
Also, I'm still curious as to your thoughts on why you think we'll all be amazed at how few will utilize FP+. I think it's a safe bet that, once Disney's Marketing arm begins driving it and every CM and TA is trained and incented to, not only push it, but also begin making those FP+ reservations for guests just like they do for ADR's right now- we'll be amazed at the opposite.