Dodge, I think there's a couple of flaws with your original post. First off, during the accumulation phase, people will be adding funds. They won't just start with $1M and sit back and rebalance until they retire. Secondly, if somebody is 100% stocks, they most likely will not be 100% US Total Stock Market.
So here's my version of this...
-Portfolio 1: 57% Total US Stock, 23% Total International Stock, 20% Total Bond
-Portfolio 2: 70% Total US Stock, 30% Total International Stock
-Portfolio 3: 35% Total US Stock, 8% Mid-Cap, 27% Small-Cap, 15% Total International, 15% International Small-Cap (basically my portfolio)
This is a starting value of $18,000 and contributing another $18,000 per year.
It's hard to tell from just the chart, but looking at the Annual Returns tab, portfolio 2 of 100% stocks is bigger than portfolio 1 of 80% stocks starting in 1978. From 1978 on, portfolio 2 never drops below portfolio 1 in value.
Now we can also probably argue that most people who are 100% stocks will not be just be a split between total US and total international. They will probably add small-cap, REITs, emerging markets, etc. Portfolio 3 takes the lead in 1975 and never drops below either of the other 2 portfolios after that.
So for me, I want to be 100% stocks during the accumulation phase (to FI, we don't plan on fully retiring early). Once FI, I could then begin to purchase bonds with future contributions/dividends if I thought I would want them in retirement. But for now, the plan is 100% stocks for life.