The book is rather grim at times, especially in the description of WFB's dependence on sleeping meds during his last sickness. His sister Carol said he needed to go to detox, but then Christopher makes an interesting observation: that great men, like Evelyn Waugh who had a similar problem, are used to being in control. Need to be in control. They are definitionally not "let's just go with the flow" sort of folks. The drugs offer them that desired control, over themselves or their disease.
I think we're all control freaks now, to some extent or another. The sort of Greek tragedy in it it seems to be this: that the very thing that allows for lingering, painful ends that exacerbate our lack of control - modern progress, in this case medical progess that extends our lifespan due to wonderful inventions like antibiotics and blood pressure meds - is also the thing that fails to train us for our end, or makes us prepare for it in the worst possible way via the technological comfort that gives us a constant illusion of control.