> Something needs to be done about young children accessing pornographic content, and terrorists and paedophiles using end-to-end encrypted chat systems to cover their tracks
When you come up with an approach that doesn't involve breaking the system for literally everyone else, please let us know.
There isn't anyone who would dispute that these issues are serious but hammering backdoors (of which clientside scanning is effectively one) into encrypted chat systems is not and never will be the answer.
It is not the case that if you put a backdoor into WhatsApp that the criminals suddenly have nowhere to go. They will simply move to lesser-known and possibly even more effective platforms instead, ones that are outside the scope of regulation. Perhaps they'll just build their own. It isn't exactly difficult to do — all of the necessary pieces are out there as open-source.
What we absolutely cannot accept is that all of the mass-market platforms become broken to the point that everyone else, including the less technical users who don't know about the alternatives, can no longer trust them.