The other takeaway from this story is that the dayhike, while physically possible, is a very serious undertaking that is prob appropriate for only the most fit and well prepared hikers.
My wife and I are in EXCELLENT shape (I have run over 500 miles this calendar year, she runs half marathons etc, both spend 1 hr plus/day in the gym), we did all the suggested training hikes, etc AND WE BARELY MADE IT. Frankly I think we got lucky, and my obsessive planning and research helped us avoid some of the typical pitfalls.
Virtually all of the day hikers that we met at the portal and on the trail didn't make it. I'm not saying that day hikers dont make it, but my sense is that many of the accounts you read about are from seasoned hikers who have extensive back country experience under their belt. I could be wrong.
My advice, take a hard look yourself, and unless you consider yourself an expert, do the overnight hike. One barrier to the overnight for us was the gear. I didn't want to buy a ton of cold weather equipment when we live in Socal where we would never use it. A couple of overnighters told us how most rent the gear online or from businesses in lone pine (duh).
I would disagree with your statement stating season hikers with extensive back country experience.
I was able to summit Whitney on my first attempt (day hike)I only came across two individuals that did not make the summit one a day hiker and one an over night hiker.
I've been hiking for about a year.