Pointless debates over legality and succession
So much of the discourse around the Dance and the main series to a lesser extent is pointless discussions about who is the rightful heir, what the law is, if the king's word is law or not, who should be king according to primogeniture, cognatic or agnatic-cognatic....
It really doesn't matter as much as some fans think it does. The IT isn't some post-enlightenment nation state. The rightful king is the one that can "enforce his claim". This is not to say that symbols of legitimacy or ancestry does not matter, rather that it's not the end all be all of succession.
The Dance is obviously inspired by the the Anarchy, where Empress Matilda, only surviving child of Henry I, was usurped by her cousin Stephen of Blois.
Charles IV, the last Capetian king, died in 1328 without a direct male heir leaving his kingdom in a succession crisis. His nearest male descendant was his nephew Edward III, King of England, through Isabelle line. However, the French nobility preferred Charles patrilineal cousin Philip of Valois instead, claiming that women could not inherit the Kingdom of France (later justified as the Salic Law), and thus Edward claim was not valid. This was a continuation of the precedent set in 1316 when Philip V took the throne over his niece Joan. This one of the main causes of the Hundred Years' War. I'm sure many of you can spot the similitudes with the great council of 101.
I could go on and on, point is : History is ripe is with succession crisis, civil wars and usurpations. It's not enough to claim that X is the rightful heir by virtues of Y law or because king Z said so. Westeros does not have the modern institutions needed to enforce strict succession laws. It's a mix of tradition, precedent, blood ties, whatever the kings decides, whatever his vassals wants, and mostly whoever has a big enough stick to enforce his claim.
This isn't just about Rhaenyra and Aegon. Robert Baratheon had no serious claim to the throne (especially considering that Viserys and Daenerys were still alive). Stannis glazers too like to think they can win any arguments too by repeating that he is the Rightful King™ by law and that Renly was a usurper that would have set a bad precedent or some shit.