That is a good question, right now CX operates varies the equipment by season, with 747 used for about half the year (presumably demand is too high for the 343 that is used the rest of the year). Having F service on some months and not in the others is one possibility (this occurs now on some routes where it varies by flight or even day of the week). More likely that ther will be no F to JNB less than a year from now. SQ is considering dropping F on JNB-SIN due to low yield.