If Miners leave the system, then the computational complexity goes down for verifying blocks. If the price of Bitcoin goes up (which it must with greater usage), then mining becomes more profitable, and will attract more minors.
Really the system works pretty well to balance itself, and likely has nothing to do with a possible crash in price.
If you look over at LiteCoin, a much less popular crypto currency derived from Bitcoin, they just has a large run up, then dropped over half their value. But in the end, they still didn't fall back to their original run up price. They are likely to continue to rise in price as compared to other options.
Really, I think the fears of Bitcoin crashing are overblown. Right now Bitcoin has a capitalization of almost 800 million. Quite a bit, but nothing like the 68 billion capitalization of eBay (PayPal). At LEAST a billion or two of that can be contributed to PayPal. And people think Bitcoin is over valued at a fraction of that? When Bitcoin arguably better addresses the same problems of facilitating transactions, only better, cheaper, more securely?