TL;DR: Cash is king — and bonds are not cash.
I've written about this in this other answer. Here's part of it:
[...] inflation aside, why do we want our "equivalent to cash" position to
be relatively liquid and principal-protected?
When it comes time to rebalance your portfolio after disastrous
equity and/or bond returns, you've got in your cash component some
excess weighting since it was unaffected by the disastrous
That excess cash is ready to be deployed to purchase equities and/or
bonds at the lower current prices. Rebalancing from cash can add a
bonus to your returns and smooth volatility. If you have no cash
component and only equities and bonds, you have no money to deploy
when both equities and bonds are depressed. You didn't keep any
powder dry. And, BTW, I would personally keep a bit more than 3% of
my powder dry.
I'll add that: Yes, one could in theory also sell some bonds to buy some stocks — and if the bonds aren't also depressed in price, then that could be a sensible move.
But! One should only sell bonds to the extent that it is a rebalancing from bonds into stock, i.e. back to the pre-crash target weights for each in the portfolio that originally made sense and, presumably, still make sense.
I think you're right to suggest that changing one's stock/bond allocation (as in: changing the target weights for each in the portfolio) could be foolish. But if stocks dropped, say, 50% while bonds held most of their value, then one has probably become overweight in bonds and could rebalance some into stocks. Some.
I'll add: When could cash get replenished? If you're still in your accumulating phase of life, cash can grow when you make subsequent contributions. And whether or not you are still contributing, your investments would likely yield interest, dividends, or other cash distributions. Otherwise, reallocating back to some cash might be something scheduled for when the market has recovered some, e.g. say when your portfolio gets back to pre-crash levels.