Consider
class A
{
public:
virtual void foo () = 0;
};
At this point it is absolutely obvious that A is an abstract class and will never be instantiated on it's own. So why the standard doesn't demand that automatically generated destructor must be virtual as well?
I ask myself this question every time I need to define a dummy virtual desctuctor in my interface classes and can't see why the commetee did't do this.
So the question: why generated destructor in an abstract class is not virtual?
Because in C++ you don't pay for what you don't need, and a virtual destructor adds overhead (even in already polymorphic classes) that isn't needed in many cases. For example you might not need polymorphic destruction and choose to have a protected destructor instead.
Further, as an alternative scenario, imagine that you have a class with a virtual method that does desire polymorphic destruction. Now imagine that the other virtual method is no longer needed and removed but polymorphic destruction is still needed. Now you have to remember to go back and add a virtual destructor or suffer undefined behavior.
Finally I think it would be hard to justify changing the default virtualness of the destructor (and it alone) based on whether a class is polymorphic or not rather than always and consistently making a destructor non-vurtual unless requested otherwise.