The Never and Future King

It seems to be taken for granted in the UK that the popular Prince William will one day become King, and that his expected child will follow him to the throne in due course.  But curiously, there seems to be almost no speculation on a possibility that could upset this scenario.

Queen Elizabeth, unlike her younger sister Margaret who drank and smoked her way into a relatively early grave, appears to take good care of herself and to be in excellent health for a woman in her late 80s, and given that she presumably gets the very best medical care available, could go on to match her mother by living to 100 or beyond.

Prince Charles has those same genes for longevity plus more on his father's side - Prince Philip is now in his 90s and still in good shape for his age.  Charles is already older than any previous heir in waiting, but there is no guarantee that he will outlive his mother.  Men tend to have shorter life expectancies than women, and there is always the possibility of accidental death - say a fall from a horse, or a plane crash.

So what if Charles were to die first?  Then by tradition the crown passes to the eldest surviving son of a deceased monarch, followed by the eldest daughter if there is no surviving son.  So instead of succession passing to William, the Queen's second son, the not enormously popular Prince Andrew, would be next in line to the throne, followed by his two daughters.  With Prince Edward and Princess Anne and their children also in the line of succession, William would move a dozen places down the list.

What would the British people make of that, I wonder?