A mechanical failure leading to a fatality, esp on an older car is almost always the owners fault, regardless of MOT. The MOT is there to ensure these problems are fixed. The problem is not everyone is mechanically savvy and not everyone has the time to give up their car when the garage can take it etc, people have lives to get on with. If you at least have a failure notice to prove the car went for its MOT before the current one was up, you can't call irresponsiblity.
What if there was mechanical failure on the car the day before the MOT was up and you killed someone? Would you be happy becasue your MOT is in date? Would you claim it wasn't your fault? What if your car failed its mot and it indecated brake failure, and you crashed becasue of a suspension failure?
It's an odd system, that's for sure. The new electronic system is there to link up to the tax and insurance systems so the DVLA know who the people are without tax, MOT and insurance etc, keeping the roads safer than aspect as its now very difficult to dodge tax and an MOT.