I did a quite of bit of research on the changes in society in South Africa at a university in South Africa for several years.
I recieved a tenured position at this unversity, I had enough funding for research and new projects, but as soon as Mugabe came into power, I returned to Germany.
Now here's what I think about this whole mass immigration as replacement work force policy:
South Africa has been a "multicultural society" since its beginnings. You have the Khiosan peoples who were originally there, the Bantus who came down from Central Africa in the 1500s, after 1652, the Dutch began to settle first as an outpost, in the Cape. French Hugenots and Germans arrived. With the English came the Indian indentured servants - these are both Hindus and Muslims.
After the discovery of gold and diamonds, as well as the pogroms of the mid-1800s, you had a large Jewish population from Europe that settled in the cities (especially Johannesburg).
Now, these people interacted with each other on a daily basis, they lived in separate areas, but all of these different religions, languages, cultures were existing along side each other.
Blacks roughly make up 80% of the population
Coloureds (a mixed group) 8%
Whites 8%
Indians 2%
Despite the overwhelming majority, there has been over the past 20 years since the abolition of apartheid, not very much mixing between these groups.
I don't believe we will be bred out by these new immigrants. Mixing happens when two groups are socially seen as equals, this eventually became the case with the Vikings in Anglo-Saxon England, thus they disappered as an independent group.
The most that will happen is the the whites leave areas in Europe. Perhaps for example, East Germany will secede, the whites move there and Hessen becomes some kind of Germanistan.
If anything I see Germany fragmenting into semi-dependent or independent areas however, I find it very hard to imagine that the white people will "disappear".