You suggest that to make you feel happy I should spend several thousand pounds (and still not have a certified TCAS system that will help me). I suggest if you want to stop bumping into me, or many other gliders, in cloud or in VMC, you spend a few hundred pounds. Alternatively, (since my carrying a transponder is not a panacea) you only fly IMC in controlled airspace where you will have a full radar service to ensure your safety. As Prof. Chris Reed tactfully points out, some pilots require others to spend large sums of money (but do nothing thermselves) to minimise their own risk.
You make a reasonable point.
However some elments are incorrect.
A transponder "gives" you better than TCAS. More correctly AT can provide you with a RIS. Even on busy days if you tell them you are IMC you are likely to get a service. You and others can buy PCAS - an effective on board form of TAS at the cost of a few hundred pounds.
It is not true some pilots require you to spend money without an equivalent committment. In the same way powered pilots have bought a transponder.
The reality is our regulatory authority has based collision avoidance on transponders not FLARM. You might not agree, but there is little sense in people using different systems.
I agree the apparent lack of risk is a consideration. However, statistically an accident will happen. The consequences are horrific.
The number of aircraft and pilots that are "qualified" to operate in IMC is relatively few - it seems extraordinary we cannot agree on a common system for these few.