Her extreme longevity must put her son, Charles, in a lifelong difficult and awkward position. (Not that I'm any big fan of his, from what I've seen and read.) But whether you like him or not, there's something strange about a man living his whole life in the expectation that he will someday be king, and if it does happen, he will obviously be quite old (and ready for retirement! And to my American mind, there's something vaguely creepy and archaic about the whole idea of primogeniture.
Of course, the political power of the UK lies with the Prime Minister and the elected Parliament, so the whole royal thing by now is mainly symbolic and emotional, or so it seems to me.
I'll probably never have the real answer to my original question, but I'm sure there are reasons within reasons for her hanging on. We'll never get the real answer. (Unless Harry and Meghan decide to write a book?)