The closest thing I could think of is if someone finds a way to make gold to where it would be impossible to tell between the natural and synthetic gold.
That has already happened. A number of Hong Kong gold dealers were fooled into buying fake gold bars last year. The forgeries were clever alloys of metals such as osmium, tungsten, silver and coated with gold to have the same density, size and weight as a real gold bar. The bars can be made at less than half the cost of real gold. Older forgeries consisted of tungsten coated with gold, but this new approach is harder to detect.
Truly synthetic gold can be made in particle accelerators and nuclear reactors from mercury, but it's more expensive than natural gold and also radioactive. Not very useful and easy to detect.
Could bitcoins suffer a fatal security flaw that makes them useless? Possibly. It's all open source and apparently secure, but how many forum participants have downloaded and analysed the source code? Not many, I'm willing to bet.
As to why I have never purchased a bitcoin (apart from trading and using proceeds of mining to buy bitcoins), the answer is pretty clear. I have yet to find a product that cannot be purchased easier, faster, and most importantly cheaper in bitcoins than with a conventional currency. Merchants automatically have to incorporate a large price cushion into their products to help counter the massive volatility of the exchange rate. Since the products they source are purchased in conventional currencies there is no current way to insulate bitcoins from the real world.
Bitcoins are currently at $8.27 and falling. My purchase orders at $7.1 look like they'll be filled soon. Not to use bitcoins, but to trade them on an up tick.