Realistically think about it? How can they hold back fastpasses for day of when demand far exceeds capacity for those rides? If they do that then somebody making their FP appointments a few weeks out may not get them. How do you think that will be perceived?
It's going to go the same way as ADRs, first come first served. If you miss out, then too bad. I cannot foresee it going any other way.
As for the current day of, it's completely possible that when that happened they took those from the allotment of FP- for the day and made them available to FP+. However, when there are no FP- to take from, then what does Disney do?
And FP+ being there the day of goes directly against EVERYTHING Rasulo said, which is you lock the guests in as far in advance as possible in order to get more money out of their wallet.
I understand your concerns, but FP is different than an ADR in terms of capacity (and demand). So the FP/ADR analogy can only be done very loosely. It is apples and oranges.
FP capacity has always been an issue with the rides you mentioned, but maybe Disney believes that FP+ and holding back a certain percentage of day-of slots is a more effective distribution method.
I will say again that while Disney is certainly in the business of making $$$, it knows it can't make money if the guest experience is diminished significantly.