Maybe I'm dense, but I don't see the problem with keeping the term pentathlon for the modified competition in which shooting and running are combined into a single contest. While the modifications result in a reduction from five to four events, five sports continue to be involved. Nor is counting sports rather than events an innovation. The biathlon, which combines shooting and skiing, is a single event in which skiers stop at intervals and shoot. The term biathlon, to which, as far as I know, nobody objects, provides precedent for the interpretation of the names of such competitions as denoting the number of sports which they combine rather than the number of distinct events. There is no reason to treat this as a case in which a word's meaning has ceased to correspond to its etymology.