Porfolio allocation helps reduce risk. Time helps reduce risk. Once retired, you no longer have the time to earn back losses, so the risk of retirement funds running out rests squarely on allocation.

Risk (to me) is defined as maintainance of principal balance. I do not want to see account worth $1.5 M in year 1, only to lose 33% and be worth $1 M a year later (even if it could also be worth $2 M a year later).

At retirement I plan to have positions in:

Cash/Bonds (25%)
Large Cap Value (20%)/45
Large Cap Growth (10%)/55
Mid Cap Growth (7%)/62
Mid Cap Value (7%)/69
Small Cap Growth (5%)/74
Small Cap Value (5%)/79
International Large Cap (15%)/94
International small cap (6%)/100

The longer one could keep an allocation like this, the better they will be. As previously stated, the 25% cash and bonds is a retirement position, with ~7 years worth of income. As this gets spend, the portfolio will become "riskier" as cash is reduced. Riskier might become as much as 85%/15% Equity to cash mix.

The way to reduce this risk is to sell off the sections with highest volatility (volatility is change of principal balance). So the International Small Cap and Domestic Small cap positions will be eliminated first.

Then the mid cap positions get reduced.
Then the international large cap positions get reduced.

To point where a 60-40 portfolio looks like:

40% cash/bonds
20% large cap value
10% large cap growth
10% mid/small cap
20% international