Perhaps my explanation was lacking so I will give it another go, I'll take the blame on this one.
Let's take Soarin' and lets say that under normal circumstances that the rider capacity is 2000 guests/hr. So 1000 people get through in 30 minutes. Now we add in the concept of Fastpasses over the last decade. So if 1800 people are in the stand-by line for about an hour wait, let's say that 200 people are able to use FP for that same hour. So a total of 2000 people are now in line for Soarin'. The FP guests are shuttled through the ride more quickly then the stand-by guests and experience significantly shorter wait times. But the entire 2000 guests are still moved through the ride within 1 hour. But the benefit of the FP was that those guests experienced a wait of less than 15 minutes. And we can all agree that is the #1 reason we like to use FP, so we avoid the 1 hour wait and can get on in under 15 minutes.
But this ONLY works if that out of every 2000 guests in line, ONLY 200 have FP, they reap the benefit of the FP as expected. But let's change it up and allow 1000 guests to use FP for that same hour window. And I'll even grant you that all of the new FP users came from the stand-by line so that line is now only 1000 guests, so we still have a 2000 guests for the hour. The wait time for FP has now increased as there are 5X as many guests in the FP line. But for the stand-by folks what should have been a 30 minute wait (only 1000 stand-by guests in line) now becomes 60 minutes as there are too many FP guests being served AND they get first rights to get on the rides per the CMs.
So the stand-by guests are getting false information on wait times in this circumstance, and that is taking into account that ALL of the new FP guests all came from the standy-by line. But imagine that from the first example that there are still 1800 people in the standy-by line and then you add in 1000 FP guests as well. Well for the stand-by folks what was close to an hour wait would become 90 minutes because of all the FP guests.
You can see that as you add more and more FP guests that not only does it make the wait for FP longer that it really negatively impacts the wait time for the stand-by line. And folks look at wait times to decide if they want to grab a FP to come back and ride later or are they willing to get in the stand-by line now because the wait time is short.
Now if we change to what you like which is give everyone a FP then lets move all 2000 guests to the FP line. Now what does this accomplish? It makes the FP wait now 1 hour long and that's if you aren't also trying to get some standy-by guests on the ride. Which begs the question, what is the point of giving everyone a FP when the same wait time can be acheived if FP is completely eliminated and everyone is back in the stand-by line with the same 1 hour wait time.
To me it's really not a complicated mathematical problem, basic managment science really. But like the
DDP made ADRs required in order to eat someplace (no more walkins), if they dramatically increase the number of FP issued then the very same problem will now spill over to all the attractions. I can safely assume that the vast majority of guests will hate it.