That's not possible. At the very least, cash cannot "go down" along with everything else. If everything is down then, by definition, cash is up because you can now buy more of everything with the same amount of cash.
Yes, if you held cash in 2007, you beat the market, but you weren't wealthier than you were the year before.
During that time period the numbers in your bank account didn't get any bigger. But if that were the sole qualification for "increasing wealth" then the folks constantly worried about deflation are totally wrong!
However, until someone seriously suggests keeping a significant fraction of your investment portfolio as cash (preferentially to e.g. bonds) as a hedge against market crashes, I don't think it makes sense to classify cash as an investment instrument.