Over 13,500 Halifax and Lancaster and 6,700 were built. Now assuming fewer heavies and more light bombers were built what might that mean?
Swop the numbers around. You could have had 1,000 Mozzie raids and presumably less losses. Your heavies might have sustained the same percentage loss and the attrition replacement rate. It would appear there would have been fewer aircrew lost.
However, faced with 1,000 bomber raids by the heavies the German air defence would have been optimised for the greater threat. If that threat had been from faster bombers at greater heights then you could assume a high altitude defence would have been a higher priority.
The defence might have evolved to include rockets, missiles, and more jets and rocket AC.