Nearly six decades after electric-guitar-based rock and roll began its dominance of mainstream popular music, the fad has finally run its course, a group of fans, critics, and record industry executives confirmed Sunday. 'At this point, who'd want to sit through another incendiary guitar solo?' said Eric Clapton, a guitarist. 'There's really only so much you can do with the thing. The guitar has had a nice run, but it's time to move on.' Confirming reports that six-string sales have plummeted, music store owners noted that business has been steady overall, with teenagers eager to put down $500 in hopes of riding to stardom on the new clarinet craze. 