While I agree with your point somewhat, the scale is completely different. Its not a black and white thing. I know a lot of people will disagree with this statement though.
The main issue in preventing child abuse is that there are simply not enough resources allocated to handle each case - especially in a timely manner.
More surveillance just means more cases in the backlog, so contrary to what people wanting to implement it are saying, it doesn't get us any closer to the goal.
I know here in Illinois, USA if your parole specifies treatment (e.g. sex counseling, criminal behavioral thinking counseling etc) then you can voluntarily skip all that by just spending half your parole time in prison and leave with no restrictions and no therapy.
This happens to a lot of inmates involuntarily because the prison system can't find anywhere to house them, so they are forced to keep them in prison and then release them with no help and no support.
I don't see how scanning for the material in major messaging platforms would impact anything. The people will just move to the next service that is encrypted. The technology exists. It can not be banned.
More surveillance just means more cases in the backlog, so contrary to what people wanting to implement it are saying, it doesn't get us any closer to the goal.