The traffic potential is really a big question.
Both MYS HAS and YPR HAS routes have very low passenger traffic, and HAS is pretty much the only station on the route that has substantial passenger traffic.
YPR SBGA MYS would be almost double the distance compared to the shortest route and is also a single route. The travel time would be at least 2-3 hours more than the Mandy route. And regular travellers between SBC MYS would never prefer this long journey...
more... when most buses compete the journey in much lesser time.
The only train I can think of, that 'may' enjoy some patronage is an ONT to MYS, but even that is doubtful.
The proposed route via SBGA or the existing route via HAS can never serve as dedicated SBC MYS connections as they are just too long to act as alternatives for a 3 hour journey.
If decongestion of the main line is necessary, then some of the weekly long distance trains from MYS can be rerouted via HAS-SBGA. And in this case, HAS is the only station that can provide a reasonable amount of passenger traffic, so it is better to run the train using the existing MYS HAS and HAS YPR lines instead of the proposed shortcut.
Bottomline is that there is no real benefit of opening this shortcut.