Part of the issue is that there isn't an automatic requirement for medical professionals to notify the DVLA of someone's unsuitability to drive. They can do it voluntarily - but there's no process whereby they are required to.
It should be an automatic process that the DVLA are notified when someone is diagnosed with something notable. Relying on the person doing so when they have so much to lose and an inflated sense of them being a 'safe driver' is such a flawed system.
For example, there was absolutely nothing forthcoming from any of my father's 5 consultants to do with his MND care or his GP to the DVLA about the potential for his driving to need to be evaluated. This is a degenerative disease that massively affects strength, co-ordination and control. To my father's mind this meant no one was concerned about his ability to drive therefore he was allowed to (we didn't let him).
This shouldn't be an individual decision, it should be a mandated and assessed properly - if you diagnose someone with something concerning for the ability to drive you're legally required to report it to the DVLA for investigation.