This is the problem with letting juries decide.
Since the facts are often clear-cut, it should be a simple and straightforward case of finding that the accused was driving the car at the time, or wasn't. Judges then take forward the sentence based on the consequences (i.e. nothing happened, or 20 people died - whole different ballgames). It frankly shouldn't be possible for people to evade sentencing on causing a death when it's a simple fact that a death was caused.
Even the vision problems of the accused are not relevant to the jury - that's just an example of an aggravating/mitigating factor for the judge to use in sentencing (in this case, the former).
It should be a simple case of saying "ok, there has been a crash and somebody is dead. The Crown say that the driver of the vehicle was Mr Bloggs - proven or not proven".
You wouldn't get the stupid situation where the defence say "would you really want my client, of previous good character, to go to jail? I ask you to find that he was not driving his car and the identical-looking chap caught getting out of it on CCTV was actually someone else".
Then the judges could get on with sentencing according to a much wider range of parameters - in this case, causing a death (maximum seriousness) and driving when blind (also maximum seriousness).
That doesn't mean actually that I think it's appropriate to send the driver in this case, a septagenarian, to the slammer for the rest of his natural. Instead, removal of the driving licence and some sort of community service would seem fine. If you want to punish anyone, those who tested his eyesight and did not contact the DVLA on his behalf to have his licence revoked should be hit with big fines.