You and others making this claim that people knew or that it was obvious are the ones lacking common sense. I'm literally astounded at the stupidity on display here.
Firstly, anyone 'on their way out' must have a terminal disease. There's simply no other way that anyone can be certified as 'living on borrowed time'; and even normal people don't casually reveal that they have been diagnosed as terminally ill to any Tom, Dick, or Harry. The idea that the queen of England would be diagnosed as being terminally ill and that this would be revealed to members of the media under an oath of secrecy is utterly imbecilic.
If the Queen and the royal family were happy to reveal such a thing to members of the media (unknown to them and outside their family, their social circle, and even class) they might just as well go public with it - which is what they would do given that it would be considered of the utmost importance and in the public interest. The royal family maintains a public website, for crying out loud.
Then there's this ridiculous argument that at age 96 "it was absolutely fucking obvious that she wasn't going to live much longer!" - rarely have I heard such rubbish. People are increasingly living longer and the number of people living beyond 100 has increased vastly over the last few decades. Add to this the fact the royal family receives the absolute best of everything - in the national interest if nothing else - and every one of them is likely to live far longer than the rest of the populace. Indeed, this is the reason she, and other royals, are able to work so long and always look so good for their age. They have personal chefs and dieticians, personal medical experts of whatever type is needed or recommended, they have the best food, prepared in the best ways; they can have procedures such as colonic irrigation at the snap of their fingers if they want or need it, and a thousand other things available to them that normal people could only dream of - if they even knew about them.
So, the idea that the queen was obviously living on borrowed time just because of her age, is laughable. The fact is her health would have been monitored daily by experts (as it had been for decades) and she would have been 'as well as it was possible for them to keep her' without her actually having any specific illness; since old age is not an illness, they can't treat her for it. All they can do is feed her properly, ensure she doesn't fall or strain herself, encourage her to do age-appropriate exercises suited to maintain a healthy heart, protect her from pathogens, etc.
Now, assuming she died because of organ failure due to her age (since no one can live forever with the best of everything), there would be symptoms of this that her carers would immediately respond to e.g., shortness of breath, feeling faint, abdominal pains, etc. and her medical team would go to work to remedy everything that showed up as needing attention. She would be taken to a place suitable for this and closely monitored minute by minute. None of this need be expected at all. The doctors will of course have seen old people die hundreds of times and recognise the signs if they were there (which they obviously would have been) and that's when the first contact with the PM would have been made; but even then, they would have been hopeful of a recovery.
The obituary documentaries are prepared as a matter of routine not because they thought death was imminent but because they have to be prepared because death can strike anyone at almost any time, and high-profile figures like the queen are at a greater risk because of their potential to be a victim of an assassination not just because of their station but because they are regularly in close contact with members of the public.
I'd bet everything I own in an instant that most people were shocked by the sudden death of the queen and that almost no one responded to the news by saying, 'How could anyone possibly be surprised given her age?'.