A fair point, but as a statistician your case is what we use to call hasty generalization, it's a logical fallacy if you cannot argue about it from the data. If it is unsustainable you just to show that it appears to be the case.

There's data out there and from what I have read African's fertility rate is lowering, albeit a little slower than some people wished it would. We just have to wait some more years and see what happen. I believe Africa will follow the footsteps of every developing country until now and will have a society of greater urban population and lower fertility rates with economic growth. India and China are still in their way following this path.