Complex sentence: Attendance outpaced the first week of the American Football League in 1960, averaging just under 43,000 a game, but the box office numbers proved to be the beginning of the WFL's undoing.
Split into simpler sentences: Attendance outpaced the first week of the American Football League in 1960, averaging just under 43,000 a game. The box office numbers proved to be the beginning of the WFL's undoing.