I'll contribute my thoughts based on my knowledge of my DF's Alzheimer's.
It's a disease which impairs memory, thinking and personality. And in my experience is usually well advanced before a diagnosis is obtained, meaning that driving skills and abilities may have been adversely affected before a diagnosis. Alzheimer's diagnoses are often preceded by Moderate Cognitive Impairment, so those people are not at the top of the game, and the nature of the problem means that they might not be aware of their increasing deficits.
My DF was 80 when the signs of dementia first became obvious, but it wasn't till about 5 years later that he was diagnosed with Alzheimer's. I see from the Govt website that this means he was required to advise the DVLA and his insurers of this diagnosis - and here we (speaking generally) hit the first problem. My DF, like many others with dementia, forget that they have a diagnosis, and/or are in total denial of their symptoms and how they are affected/declining. Someone would a diagnosis of Alzheimer's would either not fill the form in at all, because they don't remember their diagnosis, or fill it in inaccurately, not out of a intention to deceived but because their illness means that they are no longer a reliable witness to their own life!
I would add to this that it's common for spouses of those with dementia/Alzheimer's also to take refuge in denial. My DF had already stopped driving before he was diagnosed. I'd like to think my DM took on the driving after I had refused to let him drive with my DS in the car, and I also refused to be driven by him. His judgement was impaired, and although he didn't have any accidents, that was largely down to luck and probably also the alertness of other drivers around him. My DM is one of those spouses who took refuge in denial of my DF's very obvious decline, so I suspect something really scary must have happened when he was behind the wheel for her to take on the driving.
It's my opinion that as soon as someone is diagnosed with Alzheimer's or any form of dementia, they should surrender their licence. Their decline can be rapid and unpredictable.
I'd also say that anyone who is deliberately driving more slowly because they're aware that their ability to react quickly is impaired, should also stop driving.
And passengers who don't drive, aren't always aware of the difficulties and problems inherent in driving.