What value has gold? you can't eat it. it doesn't give you warm. The only reason gold is valuable is because of its role in the monetary system. So this is just a convention. The reason gold was chosen for this was basically that it isn't as easily available, the mining process is slow, and there's still enough gold to run an small national economy in 1000 BC.
Gold has intrinsic value. It's rare, it's shiny, and people enjoy owning it. It was the money of choice (choice being the key word) for thousands of years. Absent the coercive, conventional demand for fiat money, few people would value pieces of paper featuring portraits of funny-looking dead men. Fiat money has no intrinsic value.
So basically, Gold is just valuable because everyone agrees it is.
Similiarly, the dollar is only valuable because everyone agrees it is.
We need currency. The actual medium is not important.
You got one-of-three right. Gold is valuable because everyone agrees it is, absent coercion, or convention which dictates its acceptance virtually everywhere you shop.
The dollar (which is not really a *dollar as I will explain later) only has value because legal tender laws guarantee its circulation, by penalty of physical violence. The only reason everyone "agrees" to accept it is because it is conventional, that convention arising ultimately from the coercive force of taxation.
We absolutely need currency, but the medium is crucially important. We can choose a currency which has physical limitations on the expansion of its supply by greedy bankers and politicians - gold and silver, or we can submit and accept pieces of paper with values decided by a few men, for their own gain.
The idea that we must accept fiat currencies to stave off economic depressions which are in fact created by fiat money/fractional reserve banking regimes is total bunk. I suspect this mentality has been promulgated by schools endowed with lots of fiat money.
However, when the economy is doing good, more people do the same investment. You will no longer be able to sell it for much more than what it costs you to produce as the prices drop due to concurrence. Ultimately, you won't get anything back from your investment. That's the point where people stop investing (or actually, even before that due to the so called Liquidity Preference) and money stops turning around. The turnover rate drops, and the national bank has to increase the money supply (bubble style) to keep the prices from dropping (Deflation)
You're conflating the Law of Diminishing returns and economic recession or depression. This law basically dictates that the rate of profit will trend downwards towards the average cost of capital, and has nothing to do with overall recessionary trends, or causes.
Given a static, constant money supply, productivity increases will result in an appreciating currency and lower prices (the productivity norm). Somehow you're trying to cast this as a bad thing. The ability to purchase more goods and services per unit of currency is a good thing.
Depressions are usually caused by sharp monetary contractions. The Great Depression was preceded by a one-third contraction in the money supply by the Federal Reserve, as noted by Milton Friedman. The contraction itself was preceded by a fiat-money induced expansionary boom during the roaring twenties which resulted in malinvestment, and stock speculation.
The enemy of economic expansion is not sound money, but fiat money and fractional reserve banking regimes. The cause of the problem is not the solution.
* A dollar was defined by the law of 1792 as the Spanish milled dollar which contained 371.25 grains of fine silver. By this standard we can see how much it has depreciated. To whom did all of this lost purchasing power go?