Hang on a second here. Passive traffic monitors will NOT be obsoleted by ADS-B.
ADS-B will be most effective when in range of ground stations that can provide TIS-B and FIS-B. If you install a UAT, you will have not have traffic coverage outside the range of these stations. A passive traffic monitor, however, will work in areas of radar coverage that don't support the uplinks (mode A and mode C) and will pick up the 1090ES transponders who are constantly pinging even outside of radar coverage.
Hence, passive traffic monitors have a transitional roll, plus an ongoing roll for UAT (as opposed to mode-s) equipped aircraft.
HOWEVER (this is the biggest point), when you cross the border into Canada, ADS-B is not supported (UAT or mode-s). You'll need a passive traffic monitor to help you avoid traffic.
Conclusion: You will need both ADS-B hardware and passive traffic monitors for maximum protection and flexibility.
In my opinion the Monroy ATD-300 (and ATD-300+) are the best passive traffic monitors available. Banning Jose from comment on this forum is a short sighted move, because he has more expertise than almost anyone else on traffic monitoring devices of any kind.