This is based on a misunderstanding of what predictable deflation does in a market. In fact, predictable deflation is already priced into the current price of a bitcoin. If a bitcoin is expected to be worth $25 next year, it cannot possibly be worth $10 right now because all the people who would rather have $25 next year than $10 now would bid the price up.
Your refute is based on the misunderstanding that the deflation has to be predictable. It doesn't. There only has to be a reasonable chance of it happening, and there definitely is. This makes taking up a loan for a house or even something much cheaper denominated in bitcoins a much higher risk than any sensible person would take.