I think you're missing point. For a system to be allowed on a passenger plane, it must be certified.

That means that it has to go through actual real QA (something foreign to modern Software science.)

You want to put in a glonass receiver? sure, but it has to operate independently of the GPS system, and it cannot replace any other navigation system unless the FAA allow it.

the point is each system must be robust (i mean really robust) and able to work past the extremes independently.