Fastpasses, as they exist, are actually a total failure.
One reason they were inaugurated was the expectation that instead of lining up for an attraction, people would go to food service or merchandise locations and spend money. However, after a year it was found that the only increase in sales was due to price increases and not an actual increase in volume.
Also, the Parks were designed with the expectation that (other than for parades and fireworks) a certain percentage of Guests would be in queues for the attractions at all times. Since the advent of the Fastpass, there are more people aimlessly wandering the Parks making them appear to be more crowded.
So if Disney were to totally eliminate the Fastpass, average waits for attractions would remain about the same or even go down, and the Parks would not appear to be as crowded.
Although it could be considered a failure (and Disney would never admit it), I'm not sure that it had NO effect. At least some guests are going and getting food, etc. while they wait. We certainly do sometimes (not all the time). We probably get a bit more food, instead of splitting something between too people because we have to rush. It may not come close to covering the expense of the system itself however. And it may just redistribute when people spend, instead of spending more in most of those cases anyways.
But there is also the customer satisfaction side. I firmly believe that I can do more things because I'm not spending time in the lines, so we can do things we otherwise wouldn't do, including just "enjoying it". I am much more satisfied with my trips with Fastpass.
DISers represent only a fraction (very tiny) of guests.
There are many ways a guest could have learned of the FP "loophole"
The guest might have run late for a FP and realised that late FP worked. He could have been told by a friend or someone in the know. He could have been informed by a caring guest at the FP machines. Or he could have read it anywhere over the internet, and even get to this info by mere accident while browsing for something totally unrelated.
And even if DISers represent a tiny fraction of guests, they still represent a pretty good snapshot of the guests using FPs
Using FP require at least a tiny bit of preparation and planning. Want proof ? Read the threads here and there on DIS, you'll come across accounts of guests not even knowing FPs were free and not daring to use them at all.
So you need to get information before using FPs, and if you get such information, the late return perk becomes obvious pretty soon, as it the info is all over the internet.
So I would question your "single digit" asumption. And even if we consider that DIS is a "special place on planet Mickey" if might be a safe bet to say that half of FP users do actually intend on using them later than the scheduled return time
I've said before - if it was really as rampant as the polls show, we'd see FAR MORE complaints about the lines getting clogged up at the end of the day, etc. I do NOT believe the DIS reflects the overall makeup of guests in terms of knowledge, planning, etc. We can see it when new people join the boards and say, "I didn't know that!".
In an attraction with a 2000 guests/hour capacity, open for a 12 hour park day, that's 24,000 guests. Say half of that is FP, or 12,000. If just 10% of the FP users intentionally use them late, say in the last two hours, that's 1,200 additional Fastpass users in that time, on top of the 2,000 FP users using them on time. That is a significant difference.
Sorry, don't see how that one passes the basic logic test. If you have 20k guest in the park, regardless if they are in one line or another, you have the same number of guests in the park. If FP makes them "wander aimlessy" and the park appear more crowded, that means the wait times are actually less. So eliminating the FP would make wait times increase and the park seem less crowded. If FP system helps reduce wait times by making people "wander aimlessly" that is a customer service bonus because while the park might seem crowded in public areas, guest wait times are reduced.
Actually, in theory it shouldn't have affected wait times at all on average, although because you have to defer your ride time there is a bit of change for the better at least early on.
But once you think about it, there is a sizable fraction of guests that grab a Fastpass, but then get in line somewhere else, or even the same attraction. So it does increase the wait times a bit. Eliminating Fastpass would eliminate the "wait in two lines at the same time aspect". But even that is mitigated to a point by the fact that you can't get another FP for a certain amount of time. (mathematically, if you grab a Fastpass, and then go to another line, you have a negative effect if the time you spend in that line is longer than the time it takes to get another FP, plus the time it would take you to get through the standby line of the attraction you have the FP for - a bit difficult to explain - but that's more of a problem earlier than later when your "next fastpass" times are shorter.)