Hello Teresa...my (admittedly limited) understanding of the succession in royal families has always been that it can be an untidy, messy thing when the line of succession changes from the nuclear family of the ruling king/queen to another family that is more distantly related to that currently-ruling nuclear family. It seems--especially in more modern times--that the preference is for the line of succession to stay true...parent to child, rather than parent to sibling, or cousin, etc.
From what I understand of this line of succession, there has been pressure on the oldest child-in-waiting to have children, or for that oldest child to produce an heir once they ascend to the throne. I believe there have been many instances in history where countries have had a fair bit of turmoil when a ruling monarch did not produce children, and when other family members fought to then acquire the throne for themselves or their children; this was usually avoided (though not always avoided) by the then currently ruling royal producing an heir, so the line of succession was true, and clear.
I think we accept that there were options in this particular case because we know what happened, and how well it turned out. I believe at the time that this all occurred it was a tremendous source of difficulty for the nation, that the line of succession was being broken, and for the reasons it all happened.
All of that said, it's possible my understanding of these things has been greatly influenced by the way movies portray these sorts of issues! Really though I was just trying to say that in "What if?" games, most suggestions/queries are valid ones. CC