I am very interested in these out-of-phase-flying-elephants of which you speak, and would like to subscribe to your newsletter.
An interesting thing this burden of proof.
Rather than make assertions that the Sisters of Silence no longer exist because there is no information describing their role in the 41st millennium, we might consider the problem thus:
1) The Sisters of Silence pre-date the Horus Heresy, and were active during the Heresy as the militant arm of the Adeptus Astra Telepathica.
2) Development and innovation within the Imperium virtually ceased with the Emperor's ascension to the Golden Throne.
3) In the absence of information confirming the demise or reassignment of the Sisters of Silence, one may assume that their organisation retains its original function and purpose.
I agree that untouchables are rare, but so are powerful, stable psykers, and the Inquisition has it's fair share of these as well. I also agree that both the Inquisition and the Officio Assassinorum have a desperate need for Untouchables (and Pariahs). This would not be the first instance of Imperial organizations competing for limited resources.
One thing I cannot do, based on the information currently at hand, is categorically deny that the Sisters of Silence exist as an organisation in the 41st millennium, on the basis that the fluff has not focussed on them post Heresy, or that Untouchables make Psykers look common
I would like to believe that the fate of the Sisters will be revealed in canon at some later point, and that they may well have been dissolved, wiped out or something equally distressing and gratifying. Until then it's anybody's guess.