By 5-10% chance of adoption I don't mean every basque ruler has a 5-10% chance. But that the chance for Basque top-level realms over a whole game (400-600 years) is 5-10% to adopt it.
Because this is very obviously far more realistic than Italy being conquered by the Byzantines, the Danes sustainably spreading Asatru faith in England, the Papacy being destroyed, etc.
The game is a historical sandbox. It's sad that some people's suspension of disbelief hinges so heavily on sexism, that they can't suspend it for this.
But for all the other weird shit that's halfway plausible, that regularly happens and makes the game fun? Sure, you have no problem with it.
I will reply since it appears that you are accusing me of a sexist bias here.
Counties conquered, Religions imposed and Dynasties rising and falling out of power are political changes that happened historically and ones that mostly refer to a political world. This changes do not challenge the overall structure of things and the mentality, it only changes the people who are ruling on the top. Also this things have a context and an explanation for happening in-game that is quite easy to reconstruct, for example, some character got a claim, a Holy War was waged or someone inherited a title (and we can figure out how it got to there). We can track the process of how that current situation became to be.
The same must be true for the enacting of Equal Succession, it needs to be born out of some process that explains how it became to be the norm, something that justifies how and why the mentalities changed, the most obvious example, the rise of a new faith that deems that Women and Men are Equal in dignity and not one subject to the other. That is context. But right now with Basques it just comes out of the blue with no context at all, while their Religion (and thus Society) being as sexist as ever, albeit being a pretty uninteligent strategy for the preservation of the Dynasty in power. If we transfer that to the other examples provided it would be like if the Byzantines became lords of a land without conquering or inheriting it, or England become Asatru even though that religion is dead, or the Papacy disappearing just by itself one random day.
Maybe it is because people does not know much about Basque and Navarrese History but, would have any kind of sense if the kingdom of England became Equal Succession while a Catholic Norman William the Conqueror holds the title? Or Capet France suddenly abandoning Salic Law for one that allows a Matrilineal Line of Succession while the King is also a Catholic French? Obviously not, he reason being that they were like that historically, and that kind of change would require a societal and mentality change as well. They why Navarra isn't worthy of being represented with the same historical realism that England and France are, since its historical succession is as well known as theirs?
The fun of this game is being able to revive and interact with History and see it change in a way that is realistic and with some sense so that you as a player can feel that the world is alive and that you are changing things and having an impact on it. There is no fun in random senseless things happening with no context to explain how things became what they are.