Minor detail, it prevents him from having any children. No more princesses.
Anyway, part of the reason is probably a simple way to limit the ex-heir general spam. A silly way of doing it without the limitation. Get 3-4 heirs married. Have the king ending up with an "accident". Repeat until you're at the final heir as a king. Now the family of the 3-4 heirs will show up in the royal family...
Hm, should theoretically be a limitation on the invisible children of the main heir as well then. They can still possibly be pulling a programming trick (married heir got 5 brothers, no children, king dies, bam 0-1 sons X daughters at below 16), but they shouldn't get children due to the 6 heir limit.
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