Christensen and Raynor:
(p. 55) Not all innovative ideas can be shaped into disruptive stategies, however, because the necessary preconditions do not exist; in such situations, the opportunity is best licensed or left to the firms that are already in the market. On occasion, entrant companies have simply caught the leaders asleep at the switch and have succeeded with a strategy of sustaining innovation. But this is rare.
Christensen, Clayton M., and Michael E. Raynor. The Innovator’s Solution: Creating and Sustaining Successful Growth. Boston, MA: Harvard Business School Press, 2003.
In several later chapters of Mark Pendergrast’s Uncommon Grounds, he documents how the major coffee retailers failed to perceive and respond to the threat posed to their business by the specialty coffee retailers. In some ways specialty coffee firms would seem to be disruptors. But they were neither "low end" nor "new market." Wasn’t specialty coffee what Christensen would call a "sustaining innovation"?