The execution may not always be perfect, but the concept of FP is well established in many industries. Instead of large crowds waiting in long lines for the most popular rides, you take some of those people out of line and spread them out over other rides with shorter lines or eating/shopping. This line leveling actually increases the capacity of the park by better utilization of rides and reduces total average wait times.
Disney has done this for years with Parades. People get out of line and spread out to watch a parade. This creates shorter lines during the parade. Then when it's over, people tend go to the nearest ride. This creates line leveling.
Does no one remember how mad you would get after standing in line for an hour only to have a large group push their way to the front to "catch up" to the person holding their spot? This was FP created by the masses.
You might think that wait times are much longer for those doing standby lines, but that is only because all of those people are not standing in front of you and the line is not out the door in the hot Florida sun.
The issue is, once people get use to a system, they start figuring out how to gain an advantage. They split up to get more FP,they use them outside of the time window, they try to use expired FP, etc. So now Disney has to come up with ways of preventing people from gaming the system.
Enforce time windows, use of FP will be linked to RFID and tracked electronically, you can schedule some FP in advance, and who knows what else.
The trick is balancing scheduling in advance, scheduling the day of, and being able to be spontaneous. This goes for FP and dining. But no matter what is done, those who take advantage of an option first will get the reward.
There is one BIG catch. Once a system hits capacity, no effort of line leveling works anymore. So now people get upset because there are no FP left or the standby line is 90 min. The fact is, this would be the case with or without FP, FP+, or reservations. Once at capacity, everyone else will be disappointed.