While it might be possible to change the passenger density dynamically, it wouldn't be very precise to control, because there are many things GSX does to have passengers look more like a crowd and less like an army of robots, and are:
- every passenger has a different walking pace and stride.
- every passenger randomly plays different walking cycles, and they are even played at a slightly different speed from each other, randomly.
- the time between each passenger is not fixed, but it's randomized between different min/max values, which depends on the Density slider
- if a parking position is a Walk/In, the time it takes for passengers to walk to the airplane will change depending how long the path is, and this can change wildly from a few meters, to several hundreds of meters.
The Passenger Density controlling the boarding speed is mostly a side effect but, the main reason for having it is performances, to set a value that is appropriate for the user system.
Also, with the current issues of AI Injection combined with GSX resulting in the maximum number of objects being hit, causing entire airports to disappear and fps collapsing, adding an option that might result in not being able to keep the object number under control, might not be a very good idea right now, at least until the problem will be fixed by Asobo.