I think the real fear from most people (at least it is for me) is whether or not they will add FP to almost every attraction in the way they have done at WDW which tends to make lines longer all around -- though to be honest, I'm not quite certain of the science/math behind why that is if the # of people in the parks is the same haha.
That is the danger of a MM+ backend for a FP+ frontend - if it expands to encompass more attractions than what the current system already does.
The entire thing is based on perception. MM+/FP+ was created as a solution to provide the perception of value to the guest as an alternative to traditional solutions to capacity concerns (adding new attractions or replacing/augmenting old ones). The perceived value to the guest of having a "FastPass" is worth something to them - e.g. "Hey, I'm bypassing those poor saps standing in that standby line". At WDW, the real concern is that they are significantly lacking at the guest/attraction (capacity) ratio. For FP+ to work at WDW, the Mouse is betting that a significant portion of guests we find value in taking a FP for an attraction that historically doesn't command much of a line (either it is truly unpopular like Stitch or has great guest/hour capacity like Spaceship Earth) instead of taking a FP for something popular (like a Mountain). When using your smart device app or standing at a kiosk and faced with the prospect of a return time for a Mountain in a few hours or a return time for something else much sooner, the Mouse is betting a portion of people that would want to ride the popular attraction may take the alternative shorter wait. It's all based on that perception of value.
At DLR, things are a bit different in that there is a mousetrap that was designed to help with the problem of capacity at WDW that doesn't have the exact same demands at DLR. DLR has a much higher guest/attraction ratio than WDW thankfully. So, the need to implement FP+ doesn't have to meet the same objectives.
It's important to discuss the operational impact that MM+/FP+ has on the parks if based on the WDW model. You mention the math staying constant. While it is true that FP+ doesn't alter the attractions native ability to server more guests in a given period of time, the WDW implementation does have impact to the actual and perceived operation of the park to guests.
- Higher percentage of guests/hour designated for FP+ than current system. In order for FP+ to retain the perception of value, time slots have to be in the system. To accomplish this, the ratio of standby guests to FP guests gets altered. More guests per hour on each attraction are devoted to FP+ than the previous system at WDW. This means that the standby line moves less efficiently. The actual length of the line may not have changed; but, the rate at which guests move through it slows.
- With more guests with FP's, that means a higher percentage of guests not waiting in the standby lines. This creates more congestion in the parks on the streets/stores/etc.
- With MM+ and FP+, a higher percentage of unfulfilled return times occurs as reservations are made in advance (weeks in the case of WDW) and often remotely (often when you aren't even in the park) and "life intervenes" and guests don't make it to the attraction on time. To accommodate this higher percentage of no-shows, the FP+ system has a built in buffer of issuing more return times that what it theoretically perfectly should to provide the proper ratio they desire for FP+/standby. The solution to more people returning to an attraction in a given block of time or less people is that the difference is accommodated by the standby line. Which means the standby "posted" time can wildly fluctuate. To offset this, the standby times are often reported as being much higher than the actual.
If DLR attempts to implement FP+ on more attractions than the current lineup of legacy FP enabled ones, that will also mean more people on the streets in a park where the real issues are around traffic flow patterns. Also, this would mean more real estate carved away to accommodate FP lines at attractions that currently don't have them.
I am still hopeful that DLR will stick to what the original plan for MM+/FP+ (which wasn't what Burbank originally wanted) - let the FP+ tech come to DLR for the current legacy FP roster and allow guests to pre-reserve and remotely reserve return times. They originally wanted to not participate in the "crowd steering" aspects of FP+ as it would overwhelm the already busy traffic patterns in the parks.
So, short version... the perception of FP+ changing wait times negatively is valid at WDW. Standby lines move slower and there is more congestion on the streets. Sometimes perception is reality. Which is rare with MM+/FP+ as there is NO VALUE in scoring that sweet return time for Stitch at WDW.
