It's only going to be of any use if the CAA mandate ONE type of EC and include Class G in the requirement. Also, you need to have mandated at least an aural warning device turned on with direction and range of threat in the audio. Having it on Skydemon or a panel-mounted screen in my opinion worsens the situation as folk will be even more heads-in than they are now.
There have been a number of high-profile fatal mid-airs in the United States recently. In several cases, in airspace where ADS-B hasn't been mandatory and wasn't even turned on.
There are other mitigations against mid-airs. A recent mid-air in the UK AAIB reports suggested that the 2 aircraft concerned were on different radio frequencies, so position reports would have been useless to each other. We need greater discipline on which units to work where. The report also went on to say some local pilots turn off their transponders to avoid being caught infringing - completely illegal!
2 years ago we needed a new transponder in the Club aircraft so chose to spend the extra on ADS-B 'out'. Routinely, we're the only such equipped aircraft in the local area. It's absolutely great for tracking the progress of students and hirers but pretty useless for collision avoidance. Our local Radar unit do a great job with 'Basic service but', then giving us advice on other traffic, based on our Mode 'S'.
TOO