Put your strawman away. Hoarding cash and saving are not the same thing. Currency is for providing efficiency of exchanging goods and services, not for long-term storage of capital. That's why it's called currency!
Individuals have a choice to save via investment (keeping capital in the economy) or via hoarding currency (keeping capital out of the economy). Beyond a certain amount liquidity buffer, widespread "saving" in the form of hoarding cash is a good way to destroy an economy by breaking the assumptions that everyone uses to fairly exchange goods and services using currency. A mildly inflationary currency is a stable currency because it discourages it being used for something other than currency, i.e. long-term storage of capital. You are arguing that zero inflation is enough to maintain a stable currency. You are wrong. We tried that, it didn't work.
Coming back to bitcoin. Bitcoin is a perfectly reasonable exchange currency for small transactions. i.e. you convert money to bitcoin then back out again. This is because its short-term value is fairly predictable and low-risk. However its long-term value is completely uncontrolled because there are so many unknown hoarders. And this problem will just get worse.
Bitcoin will probably last another few years and then it will choke on its own deflation. i.e. bitcoins will become so valuable that none of them will be in circulation. The hilarious thing is you will probably see this as a the success of bitcoin! Look how valuable they are! But it will have been a total failure as a currency. Thankfully anyone with some good sense sees this coming a mile away and nobody is likely to die as a result. (you know people die when large economies break right? This is not a game.)