As long as you keep the same *percentage* of stocks/cash, then you will never run out of cash. An extreme example:
70/30 stocks/cash asset allocation in a $100,000 account
$70,000 stocks
$30,000 cash
Stocks fall by 50%
$35,000 stocks
$30,000 cash
$65,000 total
Rebalance to:
$45,000 stocks...