RSBM:
There has been a great deal of discussion about the exceptional vulnerability of the baby and the dog, and some discussion of EC's health history of a traumatic brain injury (TBI) and another, unspecified condition that seems to have led her to withdraw from the corporate job market. These are all important factors, as is, obviously, the gruelling heat and lack of shelter from it on the trail network.
What I have not seen mentioned is any reference to JG's fitness and health. Perhaps there is nothing remarkable to note in his known/available health history. However, the fact that he was a 45-year-old man – essentially middle-aged, and a full 15 years older than his spouse and hiking companion – may also be a factor worth considering. At this age men frequently, yet unexpectedly (to themselves), find themselves at much higher risk for cardiac events and other abruptly occurring, potentially critical physical incidents.
JG's body already would have been under considerable strain from that sort of hike in those weather/air quality conditions, regardless of the weight of carrying a baby in a backpack/carrier, or possibly carrying an injured or ill dog, or even the emotional/mental stress of encountering an unexpected condition on the trail that made the interesting "day hike" he'd planned for his family far riskier than he ever imagined. If he had any sort of major health-impairing event on that trail, again in those hiking conditions, I find it doubtful that the sole remaining functioning adult, with or without her own health conditions possibly factoring into her own physical capabilities on that day, stood much chance of saving herself plus a baby and a large-ish dog. Especially if she'd also expended considerable energy on trying to save, or at least stabilize, him.
I don't know if a major health event such as a heart attack would show up in an autopsy after (possibly) up to 48 hours of decomp in high heat and arid conditions. This entire train of thought is speculative, but it's been running through my mind ever since I learned JG's age – especially as I know several "outdoorsy" men who have endured unanticipated cardiac incidents in their late 20s-early 30s.
Regardless of what happened to destroy this family, and whether it could have been avoided or not, there's a heartbreaking scenario involved. It makes me look back on my own family's summer vacations and "day hikes" in the high desert of UT/AZ, and also our impromptu driving tours of sections of CA and NV desert, and feel grateful that we came out with good times and lifelong memories as we anticipated, not regrets (or worse) due to tragic failures of supplies, equipment, or our own fragile (whether we admit it or not) bodies.