1. People ignorant to Fastpass, so they don't use it, leaving their FPs for those that do (go to TDL and see how FP works at parks where this group of people is very small).
2. You can hoard FPs early for later use, which is gone at WDW, and soon to be gone at DL.
That's it. If those two things go away, and I'm not sure if 1 will, as I see FP+ as making that an even larger group. "Hey honey, let's use one of our 3 FP+s to get a good seat for the parade."
The norm has changed during FPs lifespan. Rides have added/left. Crowds have increased. But a rigorously enforced FP system does nothing for you. It gives you a small number of short lines, at the cost of all other lines being longer.
To review: FP did not change ride capacity at the parks, ergo it can not impact the number of rides you get in a day, unless you are gaming the system, or using the system more shrewdly than others. That's it.