This indeed is the great unkown. We sort of have an idea on how FP+ is impacting the commando--negatively. And we leap to the conclusion that FP+ is helping the casual guest. And it might. But this requires far more anecdotal reporting. If we assume, as we must, that SB lines at rides like HM, IASW and POTC are longer now, and the casual guest used to ride those rides and still intends to ride them in the future, (without a FP+), we start with the premise that as to those three rides they will be spending more time in line, thus cutting down on the discretionary time that you assume they will gain to ride additional rides. So the question becomes, do they make up for this lost time by using FP+ at three major attractions like the Mountains. We just don't know. There is a tendency to assume that "casual guest" (or non-commandos) were ignorant and inefficient. But what of the guest who was casual, didn't use FP-, but always arrived early in the day because they were early risers. Also, assume that as causal guests, they were not repeat riders. So they arrived early, understood the basics of efficiency, and rode SM, BTMRR and Splash in succession from 9:00-10:00 and then bounced around the park at their leisure. When they got to HM, POTC and IASM, they encountered modest waits. So their total time in line for these 6 rides was minimal.
Now, this causal guest is told to use FP+, so they do. They book their FPs for the Mountains and arrive at the park early and go straight to ETWB or some other crowded attraction that is best visited early. They zip through their three FP attractions whenever their return time is, but their wait differential isn't all that great. Perhaps it isn't any shorter at all if the FP returns for those rides is around 15-20 minutes each. They may not be doing any better now than they did before. But now, when they go to HM, POTC and IASW, they are confronted with 45-60 minute waits that they never encountered before. At the end of the day, they may have actually lost discretionary time. Again, we tend to assume that the casual guest waited 75+ minutes each for the attractions that they are now FP-ing, and that they are gaining 60 minutes per ride of new-found time. I am sure that this is true for some people, but certainly not all. The math boils down to this: If a guest in the past waited an aggregate of 40 minutes for HM, POTC and IASM collectively, and now waits 150 minutes for those three rides collectively, their three FP+ have to save them 151 minutes over prior wait times in order to come out ahead. Is it possible for this to happen? Sure. For any guest who jumped into SB lines for major attractions in the middle of the day without FPs. But for casual guests who had any sense of efficiency and who didn't do that, then gaining 151 minutes might be hard to pull off.