Well, you see, that would change the behavior of the program. In order for it to resolve the vtable statically, which it CAN do, it would have to know what kind of vtable the NULL cowPointer has. And the true answer is that the program is already wrong once it accesses the vtable.

To put it another way, such an optimizer would transform incorrect programs into correct programs.

If you write correct programs, the optimization is useless since it only transforms incorrect programs. If you write incorrect programs, the optimization is dangerous since it makes it harder for you to figure out that your program is wrong.