The interesting issue here is that Bitcoin is a fiat currency, but it is backed by nothing. It's not that other currencies are not fiat currencies, but they are backed by something.
The US dollar is "backed" in the sense that if it crashes, the USA will do "something" to relieve the crash, and it's currently a currency that is needed to buy oil worldwide, and it's currently accepted as a "reserve currency". Could all of this change and the US dollar become worthless? Yes, of course but that would not happen until the US economy is completely destroyed. Most people would consider that unlikely, so they accept the fiat US dollar as a stable investment.
I suspect Bitcoin survived this long since it developed a little bit of real-world usefulness as a tool for moving and laundering criminal proceeds. As law enforcement gets better at attacking that criminal revenue chain, that usefulness is going to fade.
In a sense, the fact Bitcoin is a fiat currency backed by nothing is part of its allure: That makes it volatile. That makes it really attractive to gamblers or the professional gamblers who take big risks and call themselves "investors"
But eventually there will be a run on Bitcoin, just as there always is with any bubble. Perhaps this is just a big run. And there will continue to be runs as long as Bitcoin exists.