I'm certain that we don't know the whole story. It's never just one, or even two things that cause fatalities in the woods. It's cascading issues, some of which you have no control over.
So they set out to do 15 miles at 0800. At even 2 mph they'd be done by 4 pm. That's not unreasonable. Millions of people do it.
But Mr Murphy came out to play. I would cut them some slack.
I'm almost 70 and I've gone out dozens of times and done more miles than that as a quick day hike. I regularly do an 18 mile section of the Wonderland Trail on Mt Rainier as a day hike. Yes some times we finish in the dark. And yes, Mr Murphy could royally screw us over. It's a chance we take.