The following is from a contemporary technology and business innovation/change management perspective.
Ginni Rometty, CEO of IBM says, "You've got to keep reinventing. You'll have new competitors. You'll have new customers all around you."
Ginni’s comment says it all. To stay in business and to remain profitable businesses change and adapt constantly. Some changes are visible, such as Amazon Books expanding to include Cloud Services. Some changes are invisible yet equally as powerful. For example, P & G changed their R&D business model from researching and developing their own products in-house to engaging in open innovation with strategic partners.
Had these companies stayed as they were focused on existing clients without looking for new opportunities, customer segments, and new ways to remain competitive, they would not be the powerhouses they are today because their early business model would not be relevant today. A popular example of this phenomena is Kodak who went from supplying 90% of USA film industry to nearly no business at all.
So, to answer the question in general terms, companies need to focus on existing and new customers. Look after today’s customers in a way that outshines your competitors to feed your business today so that you can “keep reinventing” (Ginni Rometty) to open new customer segments to secure your business tomorrow.