The only way cash savings can be wiped out is rampant inflation, currency devaluation, or the currency being no longer accepted. Generally "crash" refers to the stock market prices dropping. Stock market prices don't have anything to do with any of the above three, so you can be pretty sure that the next stock market crash won't wipe out cash savings. If anything, crashes tend to produce recessions, which can produce deflation, which makes cash worth more. (But, deflation is really bad, so central banks do everything they can to prevent it.)

We’re in guns and ammo territory then.

The bigger risk is having a large long term cash position. Large amounts of cash should have a purpose within a year or 2.

That's a bit like asking, how do you know we won't have another world war? If there was a way to know the future, we'd all be doing ___.

this is why just about everyone suggests an appropriately diversified portfolio - stocks, bonds, cash, natural resources, hard assets, etc.