I agree with much of what you say.
However, I do think the system needs specific reforms. I've worked in family law and have seen similar potential problems in my state. It's true Josh was/is the only one legally and morally culpable (IMO) for these horrific acts. Nevertheless, we should use this as an eye opening experience to change the system for the better.
Off the top of my head:
1. Once the judge determined last Wednesday that the Cox's would retain primary physical custody of the boys, Josh should have been denied any visitation, supervised or not, until he underwent and passed the psycho-sexual and polygraph exams.
2. His status as a person of interest in his wife's disappearance, along with his tenuous alibi, while having no impact criminally, should have been considered in determining custody.
3. Additionally, there should have been more than one case worker assigned, and any visitation that had existed in the past should have occurred at a neutral location over which Josh had no authority.
Would Josh still have gone postal somewhere else? Perhaps. But the system made it far easier for him by entrusting him control over length of time, place and manner in which to plan, prepare and engage in cold-blooded murder.
To brush this under the rug as an unpreventable atrocity by an evil being risks the occurrence of a similar tragedy in the future. We can do better.