People here saying that deflation is only a problem with debt based currencies are wrong.
Inflation is needed not just because of debt, but because of the growing population. With more people, come more goods being produced and consumed. If we elected to create a 1:1 economy, eventually the dollar would start deflating. This would be unavoidable unless the economy and the population never grew. We know that's not going to happen, so we need more currency to compensate for those extra bodies.
We also need it for debt, yes, but debt is just the lubricant, not the engine.
And
And even if you disagree with that, and you think that debt is the only reason for inflation, what makes you think that if bitcoin were to become a pervasively used currency, that people would not create debt instruments with it? They've already created a multitude of other typical financial instruments based on its value. The only reason why it's not a debt based currency is because we have not yet chosen to use it to create debt instruments. But if/when humans decide to establish bitcoin as an acceptable form of transaction, they will use it for debt, sure as the sun will rise tomorrow. And it is at that point, that people will see this inherent flaw and it will come crashing to the ground.
This does not apply to all cryptocurrencies, and it is reasonable to suspect that others will succeed where bitcoin fails because they will allow for an infinitely increasing creation of units.