Very possible. But with hordes of guests pre-booking FP+ ahead of time, what do you think will be available for same day offerings? Have you tried to walk-up to a popular TS restaurant in recent years? Pre-FP+, the FPs for popular rides ran out early in the day. That is ostensibly the "problem" that needed to be "fixed." But using that historical data, one can easily extrapolate that those same popular rides will "sell out" even earlier since people can now reserve their time 60 days in advance. So where are all these "same day" reservations going to come from?
Ride capacity is a constant. So the only way to ensure "same day" availability would be to hold back a certain percentage of the daily capacity which would reduce the number of FP+ pre-bookings available.
(WARNING: speculation follows)
Mark my words, WDW is not going to hold back a percentage (certainly not a meaningful or even statistically significant percentage) for "same day." They are marketing FP+ as the ability to pre-book your rides in advance. In order to deliver on that promise, they would be shooting themselves in the foot if they hold back available times. The guest who couldn't get a FP+ between day 60-1 because all of the "pre-book" slots were gone will be TICKED if the schlub who arrived 5 minutes before him on the "day of" is able to get the FP+ he tried for two months to reserve because it was "held back." At best, you will be able to choose from the picked over scraps (Stitch anyone?) and if you get really lucky, you might be able to snag a "no show" FP+ if those get dumped back into the pool.
P.S. Note the overt use of "mark my words."
You keep using that phrase. I don't think it means what you think it means.