You cannot just compare obj-c vs generics, you need to also account for the fact that when you are obj-c development, you might need some fast code and as a result drop to c. And code reuse is hard to impossible in C.

The fact that code reuse can be done without a performance hit is enough of a difference difference on it's own as it pushes down the level at which code reuse can be done. E.g. you can use generics for Graphics or Audio or general low level stuff.