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You should take a look at modern portfolio theory. It is generally accepted in the professional investment world that 100% equities is not an efficient portfolio.

But the whole point of MPT is to maximize the expected return while minimizing risk. As with MPT, one way to do that is through diversification.

But you know what else maximizes expected teurrn and minimizes risk? Increasing the time in the market., especially if one has a diversified equity portfolio (indexes usually), which is also aligned with MPT.

Let's say a person has 35-40 years until retirement, and is either going to choose an 80% US large cap, 20% US Total Bond Market portfolio or 100% portfolio for the first 30 years with $1,000 invested every month (similar to how most people invest).

Here is the historical performance of the two portfolios (80/20 is second).

As you can see the 80/20 portfolio has lower returns but also lower volatility, so as a result it has slightly better risk-adjusted returns (e.g., Sortino, Sharpe, etc). While those show the benefit of more diversified portfolio, they are flawed for the purpose of a 30 year investment with smaller contributions throughout the duration of the investing (essentially DCA). Specifically they measure risk on an annualized basis (standard deviation, downside deviation), but like any standard measure of variation around an estimator, the larger the sample, the smaller the standard error around the estimator. Furthermore, these measures don’t account for the minimization of volatility/risk with a DCA approach.

In order to see how the length of the investment impacts the performance and risk, I’ve done two Monte Carlo simulations based on the statistical returns over 30 years. Below are the expected returns at the 10th, 25th, 50th, 75th, and 90th percentiles.

The first one is 100% large cap portfolio, and the second one is the 80/20 portfolio. .

Now unfortunately it only provides the time-weighted rate of return (TWRR) at the top, and the final balance is based on the money-weighted rate of return (MWRR) given the monthly contributions (which minimizes volatility). But as you can see 80/20 portfolio underperforms the all stock portfolio across all intervals using the MWRR, and only outperforms it at the 10th percentile on the TWRR by 0.13% annually (which is 3.7% across 30 years).

So to say someone should never be 100/0 and should be 80/20, without taking into account the time horizon and the contribution breakdown, is just nonsense when the expected value is 21.4% (TWRR) to 31.8% (MWRR) higher for the 100/0 portfolio over 30 years.