Can you explain how the existing system doesn't do the same thing?
Currently, there is nothing stopping me from getting a return time on Indiana Jones, then riding Pirates when it has only a 5 minute wait and then going on Indiana Jones.
Under the proposed system, you would go on Indiana Jones first, then on Pirates.
The only difference in the existing system is you can go on the shorter wait attractions before going on the ride and in my suggested system, you would go on them after going on the ride.
So, in I don't really see the difference, other than the timing of the attractions.
Here's a few big things.
The first is that you can eliminate up to two hours (or more on the busiest of busy days) of waiting by putting your longest line at the end of the day, then leave the park. So instead of having to wait two hours like a normal guest, you get to leave immediately because... why wait until your return time is back up to leave. That's a huge advantage.
The second is that it gives a travel time advantage that the average guest does not have access to. My understanding of the 15 minute subtraction is because that's the average amount of time one should wait in an FP line (accessible car wait notwithstanding, which is a huge separate issue). So under your system,
DAS guests would get "bonus" time of any travel time between rides, if that makes sense.
An example probably makes it clearer. This has an average guest (average 15 minute FP/alternate entrance wait, 10 minute walk between attractions, no extra use of FPs on anyone's part for simplicity):
Average guest
10 - enter park, walk to attraction
10:30 - enter line A with 45 minute standby wait
11:15 - ride ride A
11:25 - exit attraction, walk to next one
11:35 - enter line B with 60 minute wait
12:35 - ride ride B
12:45 - exit ride B, walk to next one
12:55 - enter line C with 75 minute wait
2:10 - ride ride C
DAS as is -
10 - enter park with DAS, walk to attraction
10:30 - get DAS time at attraction A with 45 minute wait, return time 11
11 - enter DAS attraction A
11:15 - ride DAS attraction A
11:25 - exit DAS attraction, walk to next DAS attraction
11:35 - arrive at DAS attraction B with 60 minute wait, get return time for 12:20
12:20 - enter DAS attraction B
12:35 - ride DAS attraction B
12:45 - exit DAS attraction B, walk to next one
12:55 - arrive at DAS attraction C with 75 minute wait, get return time for 1:55
1:55 - enter DAS attraction C
2:10 - ride DAS attraction C
Your system
10 - enter park with DAS, walk to attraction A
10:30 - enter DAS attraction A line with 45 minute wait
10:45 - ride DAS attraction A
10:55 - leave DAS attraction A, get 30 minute blackout window for ride, walk to next ride
11:25 - enter next DAS attraction B with 60 minute wait
11:40 - ride DAS attraction B
11:50 - exit DAS attraction, get "45 minute blackout window", walk to next ride
12:35 - enter DAS attraction C line with 75 minute wait
12:50 - ride DAS attraction C
1 - exit DAS attraction C, get "60 minute blackout window" stamp (can enter ride D at 2, likely ride at 2:15, just after both previous systems are on ride C - the first two groups won't even get to ride D until 2:30).
In your proposed system, get to account for your travel time between attractions in this DAS window, something the average guest cannot do. Given the average distance between attractions, that's a huge advantage and a time savings of easily more than an hour over people accessing standby lines. This advantage gets bigger the more the person with the DAS is willing to walk more than 10 minutes to each attraction as well.
Combine that with the ability to "save" a huge amount of time by riding a long-wait ride at the end of the day, and you save the hour or more of travel time at the end of the day plus the length of the final ride. That's a hugely advantageous system that's right back to being very appealing to abuse or over-use.