Blenheims were the fast light bomber of the '30s. Unfortunately for them by the outbreak of the war , they were not so fast anymore.
I dont think removing the defences would have made them fast enough, so I dont see any option other than to load em up with heavy defensive armament. Its inadequate, like the german medium bomber defences, but there was no other option other than to add to the defences, was there?
Interestingly the Mosquito bomber built on this concept, and these were successful in achieving sufficiently high performance to reduce the loss rate markedly
For all its faults, the Blenheim served a purpose in those early years.