John now knows he's the true heir.
Why do people always assume this? Heir to the Targaryan Blood Line, yes. But there appears to be a widespread notion that Targaryan rule is somehow inherently legitimate. GRRM in the books and, to a somewhat lesser but still very evident extent, the show make it abundantly clear that the Targaryan kings have essentially conquered and ruled the land due to their seriously OP weaponry, a.k.a. dragons, and would have been comparable to any of the other feudal lords in Westeros if measured by any other standards. Some of them, most notably the Mad King Aerys, have demonstrably got none of the qualities of a good or even half-decent ruler.
We also see house Baratheon supersede the Targaryans after they lose their dragons and house Lannister ascending via essentially the elimination of all remaining Baratheons.
Now, if the legitimacy of rule in Westeros is based on the principle of "might is right", surely it would be Dany and her dragons or whoever happens to manage to off her whose claim would be most legitimate?
If, on the other hand, the story's resolution includes some form of assent based or otherwise evolved rulership: why does Jon's position in the Targ family tree matter?
I've never quite understood this one.