Hi Fuji, have to dispue your logic here....
(a) "No one is flying powered in IMC without a transponder. This mean RIS works."
and
(b) "90% of the glider traffic in IMC are crossing their fingers it would seem"
Given that there is (and always has been) the potential for a power IMC pilot to be in the same bit of sky as an IMC glider, using your logic in (a) then this means that (b) works too.
The only difference between (a) and (b) is that in (a) the pilot presumes that there isn't a risk (which is incorrect) whereas the aviator in (b) assumes that there is a risk (which is correct).