That would be true in a strictly hard currency world, as each dollar in circulation would become more valuable (because it would be increasingly more difficult

That would be true in a strictly hard currency world, as each dollar in circulation would become more valuable (because it would be increasingly more difficult to find another dollar to spend). However, when you take into account credit-based spending (i.e., loans, credit cards, etc.), the "limit" at which deflation kicks in gets pushed up significantly. I know there are papers/studies on this, but as I haven't had access to a university library subscription in a while, I couldn't provide sources.