About the defaltion spiral topic, the human nature of thinking is: If this thing is going up in value, I'm going to hold to it and wait until it gets more valuable
Merchants are reporting bigger sales when Bitcoin price goes up, not when it goes down. This idea that people dont spend money when its value goes up is just a myth.
That's part of the picture, current system consists mostly non-business driven speculative transactions, and these kind of transaction always goes up when the price is rising (And when price crashes suddenly), could be different when the bitcoin really replaced credit card.