If what you were saying were true, then when a line of the royal family dies out, there would be a major problem wouldn't there(?).
When Henry the eight's ligitimate children came to the throne, they were in order of succession, nine year old Edward the fourth, who died in 1553 aged sixteen, then Mary, Edward the fourth's half sister succeeded him, followed by Elizabeth the first, a half sister to both of them. When she died, and of course hadn't married or had any children, the crown went to the Stewarts, (the Scottish kings line, hence the two nations were united).
None of these occurrences could have been possible following your reasoning.