I'm booked for another week of glider flying lessons next week at a club in the Netherlands. All their gliders have a mode-S transponder fitted. So don't tell me it can't be done because of weight, panel space, electrical requirements or certification issues.
I agree that there might be some types of (older) gliders for which one or two of the problems you mentioned may be insurmountable, but that's certainly not the case across the entire fleet.
But the main issue I have to agree about is the cost/benefit issue. You can get an old wooden glider (in good condition) for less than 10 grand. Spending 2 grand or more on a transponder installation is a lot of money then. But how often do these old wooden gliders really leave the vicinity of the glider site? I'm not too worried about them flying around without transponders. It's the high performance gliders that fly competitions all over the place, sometimes in dense packs, that I'm worried about.