But they are divisible in a way which gold (or even dollars) are not: https://en.bitcoin.it/wiki/FAQ#How_divisible_are_bitcoins.3F
"A bitcoin can be divided down to 8 decimal places. Therefore, 0.00000001 BTC is the smallest amount that can be handled in a transaction. If necessary, the protocol and related software can be modified to handle even smaller amounts."
Dividing bitcoins into small micro-denominations does not solve the problems of deflation in the same way that adding zeros to Zimbabwean currency doesn't solve the problems of inflation.
Even the "leaks" from the gold supply are predictable and reversible. A lot of gold is used in electronics these days. It winds up either in recycling centres, where it can be recovered, or in landfill, where if the price of gold rises high enough it becomes commercially viable to mine the landfills.
Bitcoins cannot be un-destroyed. They are information without physical laws to guarantee their continued existence. Their manufacture and destruction are both one-way functions.
The supply of bitcoin will peak and begin an irreversible process of shrinkage.
Indeed, those who cannot steal bitcoin will be incentivised to cause the destruction of other's bitcoin. Call it the Bitfinger Scenario.
The point still stands, though, what happens when those 2.1 quadrillion units of currency get destroyed trillions at a time? Wont' we run out? Well, yes. However, when that happens we just slap another zero on the end. At present you can break a single Bitcoin into 100,000,000 pieces (known as Satoshis), but perhaps in the future we'll decide to add another 6 zeros on the end. Thus you could have 0.000 000 000 002 Bitcoins (we would almost certainly not speak of full Bitcoins by that point--there are already major movements wanting to use mBTC and uBTC as the standard units since 1 BTC is impractically large for most commerce). This maintains the scarcity model that Bitcoin enthusiasts are so enthused about while dealing with the money supply issue.
Source: http://www.reddit.com/r/explainlikeimfive/comments/1rls76/of...