There's an old saying that you should always hope for sunshine but plan for rain. That way, whatever the outcome may be, you'll be able to have all of your bases covered.
The same approach goes for showrunners on Netflix's The Crown. While they by no means intended for it to happen, creators did develop a contingency in place if the Queen were to tragically pass away during filming.
And, sadly, the showrunners have had to put that plan into effect.