One problem: in the world of "If Kennedy Lived," Lyndon Johnson is a much more obscure figure who never rises to prominence (for those who haven't read the book: LBJ quietly resigns as the Bobby Baker scandal continues to grow, with JFK's help in keeping a lid on the scandal. LBJ retires from politics and lives out his days quietly as the dean of his old alma mater, Southwest Texas State Teachers College).
Thus, there would be fewer books written about him or historical sources to detail his plans for presidency. No Robert Caro magnum opus biography which Greenfield heavily relied on as a source. So there'd be less demand, because the source of speculation wouldn't be as rich. "Kennedy is assassinated and replaced by the guy who resigned in 1964 and got replaced by Stu Symington? Who cares?"