Not to mention all the people who were not able to get a FP for that ride who would be turned away altogether.
Basically, they would be ensuring the wait is capped at 15 minutes or so, but only for those lucky enough to score a FP, by not allowing thousands of people to get in line who would normally be causing the 60 minute waits.
As someone pointed out earlier, capacity has not changed. This means that those not riding in the past were "self-selected" (i.e., those not willing to wait in a line that long). Under a FP+ only model, those who ride will be determined solely by who was able to get a FP, leaving even those who would have been willing to wait an hour "SOL."
Knowing that the ride has seats going empty even though you were prevented from waiting would be a very bitter pill for any guest to swallow.
In order to ensure they maintain capacity, they would need to increase the number of FPs distributed, which in turn would make FP not so fast.