Sad news indeed and I don't mean to sound controversial but people who have dementia can still continue to lead happy lives. They will eventually succumb to the condition, or health issues that will be caused by it, but it is not always the case that they will completely forget everyone they know. Everyone's dementia is different and some of the happiest, funniest people I've ever met have been in the care homes that I've worked in.
Obviously, the people I meet are often different from the people that their relatives remember, but it's another stage of life and I'm sure Sir Bobby will have the best of care and the continued love and support from his family.