Why input from existing customers is not enough

From the PDF article Uncovering Business Breakthroughs: Are you Tuned In or Tuned Out?, why you should get input from potential customers instead of existing customers.

Many companies create new products and services based on what their existing customers request. In fact, "customer-focused" is a strategy that many business schools teach. However, this strategy narrows a market needlessly. Existing customers represent a small percentage of the market opportunity for most businesses, so basing a product development strategy solely on what your existing customers want is flawed. Worse, existing customers have different market problems than noncustomers (buyers who don’t yet do business with you), and only frame their view of your future based on incremental improvements to their past experiences.