> Why would I buy anything with my bitcoins if a week from now they're going to be work X% more? I'd just wait until then.

If you believe this, then surely you also believe you should invest 100% of your money in bitcoins?

If not, then you believe only X% of your money should be invested in bitcoins (to avoid the risk of losing everything in a collapse)

Therefore if you have >X% of your money in bitcoins, you should spend some.

> When a currency is experiencing deflation it makes sense not to spend it. In that world, keeping my money under my mattress is the soundest investment strategy.

A does not imply B.

If the currency deflates at 1% but you could invest at 2%, your mattress isn't the most sound investment strategy.

Bitcoin is deflating at a far greater rate than you could make investing without some kind of extremely leveraged scheme.

Correct, but how certain are you that it will continue to deflate? It has already once lost 90% of its value in a short period after a parabolic rise. Could it not do that again?