Keep in mind, if BTC were the only currency then only productivity increases and workforce increases would deflate things, so deflation wouldn't be a sure thing at all times.
It will always be pretty deflationary because the population is always growing, there is going to be a hard cap on the quantity, and over time inevitibly coins will be lost at more or less a steady rate. All those things together will put pressure on commerce. I'm not saying it's guaranteed to be useless, but other currencies will be favored as a medium of exchange, IMO. Hopefully other decentralized currencies