Fuji Abound: I don't believe anyone has fitted a transponder in order to improve collision avoidance in Class G. Transponders are fitted to improve access to controlled airspace. The reduction in collision risk in Class G is a collateral benefit, but not why the transponder was fitted in the first place.
The collision you refer to will not happen "statistically" - there is a risk, which is roughly the same on average each year, though it varies with the amount of non-transponding flying (by powered a/c as well as gliders). It might happen tomorrow, it might never happen. The question is whether the costs of an avoidance measure are worth while.
As an example, there will be more than one fatal car crash in the UK today. That doesn't stop people driving. There will be more than one fatal passenger aircraft crash world-wide each year, but people still buy airline tickets. We all make these cost-benefit calculations (though not very well, beiing influenced more by the perceived risk than the actual risk).
Is the collision risk in Class G (as you now understand it) high enough to stop you flying IMC? I suspect not - just that you'd like it lowered. This takes us back to the question; is the cost acceptable, and who should bear it?
Pace asks, why not fork out for a transponder. The answer is that I don't see sufficient benefit compared to the cost. The cost is not purely financial - as an example, there is no good place on a glider to fit the aerial, and if I have a transponder fitted I'm obliged to use it (I believe) which reduces my flight time to the battery capacity I can carry. The benefit to me would be minimal - a minuscule reduction in the chance of being hit by a TCAS/PCAS equipped aircraft. I think this is the same reason Pace hasn't fitted FLARM - not enough benefit even though the financial cost is quite low (and I haven't fitted FLARM either, though it's on my list of possibles).
If anyone thinks this is dreadfully selfish, I still haven't noticed anyone on this thread proposing a measure to improve collision risk which will cost them something without achieving any appreciable benefit to themselves. I've seen a number of proposals for imposing costs on others to achieve benefits for oneself.