Off and on, I've read Swedlowe for about 20 years and, while I don't always agree with him, I've found him generally to be pretty level-headed. I think he tries to be pretty much fact based, although that means his writing is based in past data (Like FireCalc).
I had concerns in 1997 about S&P/large cap valuations and diversified. I turned out to be lucky in moving a bunch of gains to mid cap value. My "timing" was very lucky and you can't count on timing.
There is no rule that the Magnificent 7 returns can't continue to accumulate, but there are some warning signs, so I think the best thing about the article is the suggestion to diversify. I've done this since 2006, with international and smaller cap and many of you have run me in the ground. However, our portfolio did recover within about 10 months in 2001 and 15 months in 2009, so there is that. An S&P index portfolio no doubt would be ahead of ours, I suspect, by 2017 or so. The 2000 crash was a bit different, but like most of you I was just working and monthly contributing, which always works over time. I'll note that my Fidelity Contra is up more than 50% year over year, and that a good 35% of our portfolio is in it or large cap/S&P index, which has worked fantastically the last couple years. But not all of our money is there, the rest is bond funds, small/mid caps and international.Volatility is about half to 60% of the S&P but bonds and the international will swing that up or down.
Particularly if you aren't dependent on yearly withdrawals for retirement (unlike me), the S&P index is mighty hard to beat if you are thinking of your heirs or 20 years down the line. The good thing about diversification is that I can withdraw 1/2 or so from areas that have benefited, even if the S&P is down. So Jan/Feb when I do withdrawals is a bit easier. If you have enough of a stash and are withdrawing less than 3%, however, the S&P is a good way to go. We've been withdrawing 5.5-6.5%, so putting it all in the S&P seemed problematical. January I start claiming SS and the rate goes below 4% so whether it was luck or skill I'm no longer worried. I think we dodged the bullet. (Knock on wood).