The Aireon system is really designed for remote / oceanic routes where the traffic density will always be relatively low, as compared to high density terminal areas like ORD or ATL. So I don't think discrimination will be an issue for the satellites over these routes.
A bigger issue than density is total system capacity -- I believe Aireon can track about 10,000 aircraft simultaneously. But these days (on a busy day) we already have more than 10,000 aircraft flying at one time, and it's increasing every day. So that tells me Aireon will not try to track all aircraft world-wide, but will function as a supplement to existing & future ADS-B ground receiver infrastructure.
Mind you ADS-B congestion is a big issue, just not for these satellites over remote / oceanic areas.