Simple math.
Lets say 50,000 go to Epcot on any given day. Now, in the current scenario, those folks can only choose 1 of the following: FEA, Soarin, or Test Track. This means that across all three of these rides, you have a maximum of 50,000 reserved FP ride times, leaving all other openings for stand by.
If there were no tiers, they could choose all three. These means that *each* ride has 50,000 reserved ride times for FP+ now, so a grand total of 150,000 reserved ride times across all three. That is 3x as many FP+ reservations on these three rides. You can't increase the FP+ reservation number without affecting that standby lines. You have effectively taken 100,000 rides out of circulation.
Meanwhile, everyone else gets pushed into the tier 2 lines, because all their tier 1 rides are booked. So suddenly living with the land has 50k more people waiting in it throughout the day, etc....
And god forbid you are one of the people that tours without fastpasses. Because now the top three rides are booked like crazy, so you can look forward to 90 minute waits for all three of them now, even at opening, as they are booked with FP+ from open to close.
