Brewster Buffalo,
The reason that types like the Defiant, and the Blenheim and the Battle amongst others, continued in production for so long is that the Ministry Of Aircraft Production instigated a priority programme in which numbers were key. IF you had replaced the Defiant, Battle and Blenheim, to name just three, with other types then there would have been a long hiatus during which factories of Bristol, Boulton Paul and Fairey, along with their Shadow factory brethren, would have been manufacturing nothing while they dismantled existing production lines, designed and built tooling then assembled new production lines, trained up the work force and started to build the new types. That was seen as being unacceptable during the priority programme when the country was in desperate straits. To have had empty assembly plants at the height of the Battle of Britain would have failed the Daily Mail headline test at the first attempt!
FlightlessParrot,
Ironically it was the Bf110 which sounded the death knell of unescorted daylight raids by the Handley Page Hampden. Sticking to the then doctrine that the disciplined bomber formation will get through, the Bf110's merely cruised alongside the Hampdens, trained their twin machine guns in the rear cockpit on the Hampden pilot, and calmly took them out, totally unhindered by the non maneuvering Hampdens that could not bring a single gun to bear. Tragic waste.