There can no longer be "later" you refer to... If you don't buy today, the producer just won't sell tomorrow, because he will have to cut production and fire his staff... Is it really so hard to grasp?
Just because there is a fixed supply of bitcoin, that does not mean prices will have to keep going down.
If the economy is expanding, then yes, it does mean just that. The economy will be expanding with prices going down as long as the negative effects of deflation are being offset by, say, technological innovations (as it was throughout the 19th century), credit and tax policies, etc. In short, as long as producers profits aren't diminishing...
Isn't one of the problems with deflation fact that real value debt will increase?