Let's say you have 90% in stocks, the other 10% in bonds or cash, and rebalance once a year. Assume you have $100,000 in stocks, $10,000 in bonds or cash at the beginning of the year. If the stock market drops 50% (after account for reinvested dividends), you'll have $45,000 in stocks, $10,000 in bonds, for a net value of $55,000. To rebalance, you would sell $4,500 of the bonds, and purchase $4,500 of the stocks. So you would end up purchasing $4,500 of stocks at the 50% discounted value.

If you're still in the accumulation period, and saving/investing $20,000 per year with a monthly investment of $1666, over the course of the year you'll also have bought a bunch of stocks at discounted values.

At least in the accumulation phase, if you're saving 10%-20% of your stache's value each year, you'll likely fare better being all in on stocks than a mix of stocks and bonds.

Putting the numbers into the calculator at

https://engaging-data.com/fire-calculator/, assuming income of $40,000 and a savings rate of 50% or $20,000 as well as annual spending of $20,000 in retirement, then based on historical simulations, investing in 100% stocks with the above figures gives a median time to retirement (growth of the stache to $535,0000) of 11.1 years and a maximum of 17.9.

Investing in 90% stocks gives a median time of 11.3 years and a maximum of 17.9.

In effect, the continual investment of new savings provides a benefit comparable to having a portion of the portfolio in bonds.