First, anything related to default or marketplace airplanes requires the ability to install the airplane locally, since there's no way we can do anything if the content is Streamed.
Then, after this feature will come out, we might have a look at which airplanes files will be accessible or not. If they are encrypted, there's no way we can access them, even after they have been downloaded locally, so this is not changed from the current situation in MSFS 2020.
About using default passengers, there's currently zero documentation or SDK support how to use them, so we might need to experiment a bit and see what can be done, but from what I can see, they seem to be made using a very flexible method to have many different variations starting from few standard elements, but they also seems to be less fps efficient.
I also think titling this thread as "future of GSX" is a bit dramatic, considering that Passengers are like 10% of what GSX can do. In the next update, we'll focus back on Pushback, since we are still missing features we wanted to have, like Winwalkers, Push-in, Gate-to-gate push, and others we haven't disclosed yet.