Diamonds and gold make for quite shitty currencies
Gold is extremely laborious to move, and not divisible enough for most usage. Diamonds are not fungible or very divisible. Both are very susceptible to physical theft.
Network effects for money are very strong, and bitcoin has a huge first mover advantage. Any competing digital currency will require quite the innovation or distinguishing factor to gain any traction. It is becoming more 'known' and trusted with each month that passes. Pre-bitcoin-era financial groups are slowly yet increasingly giving it more legitimacy (Mainly VCs, a few funds, the occasional bank, and research groups -- also Second Market and the Winklevoss ETF are two big things to watch right now).
The main counterpoint currently is that bitcoin use is quite error-prone and hard to digest, but this seems more like a temporary roadblock to me. Y'all remember geocities and AOL right? Bitcoin will become much easier and more secure to use in the coming years. Software is relatively easy to improve, compared to gold, fiat, and the like.
There are some potential solutions to traceability, but it is uncertain how that will play out (I can't even pretend to fully understand the regulatory environments and international politics that will affect this). The one thing I can guarantee -- the way we use and conceptualize money is going to change drastically in the next several decades, and I think bitcoin will have a huge influence on this.