Not necessarily. You have to assume people will not purchase a mining unit when there's a negative expected ROI which isn't true for economic and non-economic reasons. People may expect bitcoins to appreciate and anticipate a positive ROI. Or they may value being able to participate in securing the network, the novelty or the conversational value that a flashing USB stick can have. If USB ASICs drop to $5, these non-economic factors become even larger. People will pay $10 for a humping dog USB gadget which generates 0 bitcoins, totally plausible a USB ASIC could command a premium too.

Also, your analysis only applies to the current generation of USB ASICs. Chip advancements typically allow for faster and faster chips with almost no increase in fixed unit costs. If ASICs get substantially faster (which we would expect under Moore's law) the ROI will look less shabby.