Sunk Cost Fallacy
The tendency to continue with a course of action because of previously invested resources, rather than evaluating it on current merit.
The sunk cost fallacy is the psychological trap of continuing with a suboptimal solution because you've already invested time, money, or effort in it. "I've spent six months learning this tool, so I can't switch now." This fallacy amplifies the Habit force in the Four Forces equation by making the current solution feel more valuable than it actually is. In product design, the antidote is to make the new solution's value visible and immediate, so the user can evaluate based on future progress rather than past investment. Products that highlight what users will gain, not what they've already lost, overcome the sunk cost fallacy.