TCAS provides far better collision avoidance than ADSB. It provides specific guidance on what is required to avoid a collision and if the other aircraft is TCAS equipped data links between the two aircraft to coordinate deconfliction maneuvers. ADSB does none of the above.
A very large segment of the GA fleets will not have ADSB on 1 Jan or ever under current rules. They can fly anywhere below 10,500 feet and it’s not required unless you want to enter controlled airspace.