The worry with deflation isn't that all of a sudden everyone will literally stop spending money ever again. It's that it creates enough incentive to delay purchases and removes enough incentive to invest savings (rather than stuffing the money under a mattress) to shift the economy as a whole to a lower gear. The last time the USA experienced a period of significant deflation, for example, unemployment jumped to 25%. People tending to hold onto cash is the same thing as people tending to not be willing to pay others for work.