Anyone who has flown a reasonable number of hours, even in relatively quiet airspace in Scotland, knows that "See and avoid" is not very effective due to the limitations of the eyes. So electronic conspicuity is a great thing and I have flown with a transponder (Mode C, swithed on) for ages and, a few years ago, adopted PowerFLARM to help reduce the risk of collision. Nevertheless, I think that ADS-B in/out is the way forward, especially if it becomes cheap enough to fit to everything in the air, including drones but excluding the birds!
However, the main point of such a system has to be collision avoidance imho. Virtual radar is very interesting and entertaining but, as a pilot, I really only want to know about the ones that might hit me! Ultimately, the point of ATC is collision avoidance but an ATSU also often has to maintain certain minimum legal separations so it's interests are not necessarily the same as the individual pilot's.
So I think now is the time to grasp the nettle and develop a cheap and effective collision avoidance system (probably based on ADS-B) with built in collision avoidance algorithms (similar to FLARM) and clear audible warnings of high collision risk. Mid-air collisions are thankfully rare because the air is so big but they are almost invariably terminal. It is now within our power to reduce the risk dramatically so let's get on with it!