Why are ALUs still serial?
If addition is inaccessible while multiplication is still running it is because multiplication requires also some addition operations (together with some shifting).

Otherwise the ALU needs to have more than one adder (which is not unrealistic at all). 

BTW: If the ALU can't do more than one arithmetic operation at the same time I wouldn't call that "serial" (because it reminds too much of serial adder; which performs adding operation bit by bit).
If the CPU can't do more than one instruction at the same time (in one cycle) it is just called "not super-scalar".