In Africa, OW really doesn't have much of a presence. Your only options out of JNB are London 2x/day, Doha or Hong Kong.
Star Alliance at least gets you into Geneva quicker and easier via Swiss, and 1 stop access to North America via Ethiopian, Swiss, Lufthansa, Turkish, Air Egypt, or South Africa direct.
If Air France really is attractive, SkyTeam may be a better fit since you can go JNB-ATL direct, and both ATL and DTW are Delta hubs. You're kind of stuck on options to Europe (Air France and KLM are it) but at least you have Air Kenya operating in continent.