You can never, ever rely on being able to use technology to detect all traffic as you can never, ever rely on technology to work all the time.
FLARM does look to be the ideal choice for "us" when and IF everyone starts using it
Give up hope right now then. There will probably never be a time when take-up on something is 100%. Remember it's illegal (generally speaking) to not wear a seatbelt in a car that is fitted with them - people still drive without using them. They also drive when drunk, when using mobiles without hands free kits and so on. There will always be a week link. If you care about safety do what you can, now, instead of hanging back making other people's lack of action an excuse for your own.