This route is interesting because BA now has a monopoly on it after SAA pulled out (now have to go via JNB) and VS also have withdrawn. So they can fill it pretty much for the asking and so the impetus to put new planes on is low. Additionally there is a strong demand in all classes and the 747 does actually meet the requirements for the route. CPT cannot take the A380 though it has been mooted so everything else is a downgrade in size, even the 777-300 which has 30-40 less Y seats (someone will know the exact number). So I guess its likely that the 747-400 will probably stay on this route for a few years to come!
FD.