Bitcoins won't disappear unless you lose control of the private key, or if you do something foolish like use a 'brain wallet' with poor entropy or pretend that an unregulated exchange is a bank and give them all of your money.
I am too skeptic about this whole new IT/Finance bubble. If I am wrong please correct me.
In cases where bitcoins have been stolen it is because the person failed to secure their wallet, used an insecure wallet (such as brain coins), or foolishly entrusted their wallet to an untrustworthy third party (such as MtGox).