I think that where it gets into trouble with this is that in a theoretical sense, FP+ could easily prove to actually save time. Given the following kind of average touring plan, taking the existing wait times given on MDE and using the thought that FP+/FP has doubled the standby wait times. Also, assuming the PPF/SM/Splash are my FP+ choices.
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You'll notice here why I said theoretical, because if a perfect guest return system were in place, where the guest did not have to wait at all when entering the FP+ line, then this shows that a person would actually reduce their time spent in queue, even if it is true that every attraction's wait time would be cut in half (two of the attractions above did not have wait times, or FP+, so I assumed 10 minutes, and since they are equal in both cases, it does not have impact on the final numbers).
Now, you'll see that once a true wait time is brought into the equation for FP+, meaning I do actually have to wait a small amount of time when I show up, then it can easily throw the numbers off. Even a 10 minute wait per FP+ attraction does end up having a longer wait time. That is why I said a perfect guest return system would prove the numbers above, and I agree that system does not exist.
Taken that though, I think it can easily be assumed that those numbers can be brought back by the fact that most FP+ attractions likely run at less than full capacity, meaning that those attractions, even with FP+ would likely not double their standby time. I doubt highly that rides like IASW have a 50% / 50% mix of Standby vs. FP+ riders, so more standby folks will be getting through quicker, thus reducing the standby wait time from the "double" estimate. Taking the following example, those in red have been adjusted to only have standby times impacted by 1/3 (66% of current time). This skews the numbers even more in FP+ favor.
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Just right there, I'm spending 36 minutes less of my time standing in line. Even adding back in 10 minutes waiting in each of the FP+ lines, I'm still ahead.
But,
we will never truly know the true impact of this, because we're not going to have access to the FP+ percentages, usage, and other numbers to truly prove things out. But you bet your sweet bippy that Disney is going to be THRILLED to be able to have these numbers in fairly real-time and tweak the living heck out of them now that they'll be collecting all this data.