This is a very astute observation.
And yes, this is what many of the "pro" FP+ people have been spouting since the beginning, "FP+ is way better because its so flexible, you can change your FPs on the Fly etc etc etc ... " Many of the FP+ critics have repeated pointed to the fact that this will not hold out if the system is successful. If Disney truly gets people to Pre-book, there is NOT ENOUGH CAPACITY for there to be daily flexibility. It IS NOT POSSIBLE. Now, if Disney is not successful, and people don't pre-book, then great, there will be flexibility.
The irony here is that there is most flexibility when you need it the least, during the slowest times of the year, tons of flexibility. Don't need FPs because lines are non-existent ? There are Tons of FPs available !!!!
Busy time of year ? Show up to a park and are hating the crowds and want to hop to another ? Sorry, FPs gone. Weather come in a mess up your plans and want to switch up your park days ??? Sorry, FPs gone, you can HOPE to get some the next morning (if they continue holding some back for day of, though I doubt this if they allow off site to pre-book) ... but you might also be completely shut out before you even enter or get to the park ...