having every aircraft running Mode S/ADSB would be a great solution. However as of today how many hobby and private pilots - the sort of people who would buy a FLARM or PA - actually carry the equipment to transmit it? In my experience, virtually none.
That may be true in the UK but it certainly isn't everywhere. In Germany, for one example, even many ultralights are carrying a mode S transponder. Myself will soon be installing a Trig transponder too, and mainly to be more visible to other pilots.
I might however be interested to add "something", preferrably open source, perhaps running on a RasPI or so, to display nearby ADSB transmitting planes. Flarm I will not touch even with a barge-pole - it is not under any formal norm or control, bases upon the cheapest consumer electronics, and is partially non-open.
Your bit about security holes in ADS-B is a bit disconcerting, though, I will have to look into that before proceeding any further. If substantial modifications to ADS-B are found to be in order, my transponder investment might loose a lot of its value, perhaps even all.