So what are these 70W transponders for high-flying gliders? Who makes them?
Are they even technically feasible?
The question on feasibility is meaningless. The
regulatory requirement makes them unsuitable for aircraft flying above 15,000 ft. The CAA's response is correct in that "many types of glider" do not exceed 15,000 ft.
What you need to argue for is a regulatory change to permit the use of 70W equipment above 15,000 ft by gliders. Good luck.