By classical econonic models, bitcoin has lots of weakness and is still very unmature, but if we are planning the future with a worldwide currency, every existing rules could be changed given good implementation

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

So, in order to counter this kind of thinking, there could be a devaluate design, say if you hold a BTC for more than 3 month, it will disappear

In tradional economy, each money is same, but in bitcoin world, each coin is not the same. So maybe there is a way to get more coin into exchange everyday, a mechanism to encourage people spend those coins, e.g. the exchanged coin goes up in value, while those not used coin will disappear one by one

A currency is of no use if it is not used to spend/investment, it should never be a way to save for the future, for each saved currency, there is corresponding reduced spending power thus bring unefficiency to economy, I have a very simple example can prove this