I was generally referring to any fixed quantity currency, for example gold if gold mining would stop. A market economy and financial system would obviously help a lot.
I'm still not 100% sure if saving currency is really equivalent to "investing in everything", with "everything" being a basket weighted inverse to relative prices?
Macro-economically the difference is obvious: the former would tend to raise demand for currency and lower demand for goods, thus being price deflationary (bust), while the latter would do the opposite and be inflationary (boom), but because of the fixed money supply and supposed perfect hedge, speculators can always even imbalances out and thus prevent boom and bust.
More realistic examples would have to incorporate cross border trade, which would create dependencies and invalidate conservation laws. Just as in physics, it's much easier to consider isolated systems, like an island or the whole earth, so I'll leave that to somebody else.
