Tough call ~ I agree that he has done wrong and could be said to be indirectly causing harm to children, though he never actually did anything other than watch videos. The argument that if he didn't view the images the crimes would not be committed just doesn't hold water.
On the charges of Indecent assault, rape etc he was found to be not guilty.
On sentencing we have to trust the judge. He will have taken into account the likely effect of the verdict on Langham's future career and earning power and, to be fair, he did put his hands up to the viewing charges. Remember too that in some cultures what we find abhorrent is actually quite normal and they think we are the weirdos. Some of the children on the Langham videos, if one can believe the press, seem to have come from such cultures.
Taken all in all I didn't expect him to get much more in the current climate within our judicial system.
What we can do to stop such things is another matter altogether. Frankly I don't think we ever will. Messrs Langham, King and Townsend are not the only people who have been abused in their childhood but not all go onto commit the crimes that they seem to think are excused by their own past.
Equally I have no idea how we change our sentencing or, for that matter, the entire judicial system ~ I only wish I did.