In looking at the Boeing timeline above, (Airbus similar) please note this requirement from ASA:
Note: 'accepted'...while 'accepted' by ATC, there are caveats, such as position integrity. In addition, in the US and Europe, UAT is used below 10K feet.
From a Jeppeson seminar on ADSB mandate in the US.
It is not only the GA aircraft that will feel the pain from the ADSB mandate...