In trying to figure out why a 3rd track would make some sense and why having the 3rd track as a standalone with separate load line makes sense FP+wise, I ran some numbers:
(full credit to bcrook and team on the baseline numbers)
If TSMM has an operational capacity of 900/hr with 2 tracks, that's 11,700 potential riders in a 13 hr day.
The 3rd track increases that operational capacity to 17,500/day
If we assume that Disney has been allocating 70% capacity to FP+ ressies, with currently that's 8,190 to FP and 3,150 SB riders.
Allocating 2 tracks as FP+ and one to SB (with it's own load line) gives 11,700 FP+ riders and 5,850 SB riders. that a FP capacity increase of 30% and an SB capacity increase of 46%.
But here's where it works out as a bigger benefit to Disney:
One of the biggest issues for them with the SB and FP methodologies is one of perception caused by high posted SB wait times. It exacerbates the issue of not enough capacity. The SB wait times fluctuate based solely on FP returns - when FP'ers batch up at the beginnings and endings of the return times or CM's don't follow the right merge methodology - big issues.
A static SB line with an organic flow based on ride throughput flattens this out (plus the added capacity, of course).
More importantly, they now have more FP+ ressies available to lock guests in and
more than just the organic increase caused by the 3rd track.
Now, they can go the hotel/airline overbooking route without impacting that SB wait time perception.
They have enough data now to have a firm understanding of what their FP+ no-show percentages are. And I'm willing to bet that they're higher in the morning hours and in the evening hours, just due to human behavior.
As a conservative example, if they can overbook by 35% in the first and last hours of the day, 20% the 2nd and 2nd to last hours and 15% each of the remaining, that gives them another 19-20% increase in FP+ reservation capacity.
Add that to the 30% increase created by the 3rd track, you get a 70% increase in virtual FP+ reservation capacity for TSMM after the 3rd track - locking more guests in.
If they have a higher FP+ return rate on a given day, they can dynamically decrease the day of availability (and/or, just let the FP+ line waits go from 20 minutes to 35 minutes) - no SB wait time impact.
So, there's a perception aspect, an increased capacity aspect and a virtual increase of the FP+ pool.
This would also work for Soarin' and the 3rd theater