Well, apart from the odd Click & Go, I certainly haven't noticed any decrease in the lowest fares to NBO and JNB over the last five years. On the contrary, they have increased slightly, not to mention the increase in all sorts of so-called charges and taxes. KL's lowest JNB fares were and still are some €50 higher than SA/LH, their main competitors on the route.
I'll readily confess that I do not have a comprehensive knowledge of all fares ex-AMS, so I shall ask a fares expert of my aquaintance, who has been dealing with KL for many years. Shall post any answer I get here.
Johan