South Africa is linked directly to nine European airports. Eight of them are ranked in the top 15 in Europe, by passenger boardings. The ninth (Lisbon) is ranked 29th.
Birmingham is at number 48.
You have a little while to wait yet, there are others ahead of you in the queue.
That isn't always a way of predicting directly which routes will work. Airports are there to serve the passengers who want to go through them, not just to operate routes for the sake of it. The UK and the Netherlands have the strongest historical links with RSA. Therefore it would be reasonable to ask why routes there, and to Jo'burg in particular are not operated from other UK airports.
However, the answer, as with most potential LH routes from Brum lies with looking at MAN first. Remember Globespan having a go at this too?
As Nigel says, the sheer distance alone makes RSA extremely expensive to get to, and that is before you add in the politics and the relative lack of competition. The Maldives is cheaper partly because it can be served in a relatively straight line via the Gulf - going that way to JNB or CPT is quite a dog leg.
So yes, the extension should (will?) put RSA within reach of BHX, but I still don't think that would make a route viable.