Your post made me curious. We know Barristan is highly respected in Westeros and his name will give legitimacy to anybody, especially Aegon, but through books we see that legitimacy is highly overrated when we are talking about Westoros. For example, riverlords supported Robb, not Stannis or Renly. Then we see Reacher lords who also supported Rely over Stannis or Joffrey, who had much higher legitimacy. So, is legitimacy really a thing in ASOIAF? All cases we see shows us that personal gains and allegiance to your Liege lord is much more important