DK has potential, definitely.
Too many people count Star Fox out. I have no doubt that if Nintendo EAD developed the newest Star Fox, it'd have more potential than any Metroid. When Nintendo actually made it themselves, Star Fox sold ~3mil on the SNES (more than any Metroid), and 4mil on the N64.
Anyway, the trio is Mario, Zelda, and Metroid because they're fan favorites (and Star Fox / DK haven't seen good games since the N64). It has little to do with sales, honestly.