From a purely historical point of view i would certainly agree with you (hence i clicked on "agree"). The problem remains though that the AI and the player pursue different goals. Unless the player goal changes (i.e. you can switch dynasties as long as you play as a relative) or AI behaviour changes (being more mindful to keep their dynasty in power), the game has a fundamental weak spot.
Switch dynasties, if a previous one ends might be considered, but IMHO should be bound to certain rules. A switch should diminish your score, maybe by half or a new score maybe up from half of the score of the previous dynasty and half the score of the dynasty, which takes over.
Besides, marrying an infertile spouse for prestige does not fit to the " an heir no matter what" approach you described.
How would one know beforehand? Unless one marries an older heiress, which has reached an age were giving birth to an heir is highly unlikely. Even then historically it could make sense, when one could occasionally inherit by testament or as spouse (neither is really in the game). The last member of a house would be focused on getting an heir, otherwise marrying for prestige reasons is completely valid. If due to events, the one stuck in a marriage to an old heiress becomes sole heir of his own house, you just need to bide your time.
Moreover marriages, even when both spouses are not too old, can remain childless too.
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