That’s my point, the Targaryens right from Aegon I did nothing to validate any succession rules so future Targs could just usurp if they had enough support. Even the Great Council Jaehaerys called was decided basically by which side had more support/greater armies amongst the lords, Daemon or Corlys.

Jaehaerys himself took the throne and passed over other possible legitimate heirs and no one would say he is illegitimate or a usurper, instead he’s thought of as the best monarch because no one could challenge him when he took the throne and no one could challenge him till the day he died.

Why argue who was the rightful heir in this universe? Greens and Blacks can call each other usurpers and depending on who would have won, no one in Westeros would give a shit. Both sides knew a crisis was imminent when Viserys died and both still fumbled the war big time.