Perhaps in direct attrition the Bombing campaign could be called a failure, however the very fact that it continued must mean it had some significance or impact?
The assets that the Axis diverted to deal with Bomber Command must have been significant (withdrawal of Fighter wings and numerous batteries of Air Defence Artillery from Russia which allowed the Russians to commence Offensive Ops). ISTR that an often quoted reason for the continuation of the Bomber offensive was to placate Russia as they were insisting on a Second front which we were hopelessly ill prepared for?