Putting aside the fact that this is a world with magic and divination so it’s not like you can make a claim of legitimacy and no one will fact check you on it….
What you’re basically suggesting is that I remove the current king from the narrative and the party will just have to decide who is the most “deserving” of their help due to their arbitrary views of legitimacy based on male primogeniture and their magical abilities, after which they’ll serve this new claimant as ruthlessly as they did the last….
They seem to serve the “royal bloodline” because that’s what Rupert and his predecessor preached along with the divine right of kings and for some reason the party internalized that instead of being skeptical about it.
Putting aside the fact that I don’t believe that no one is entitled to absolute political power because of who their parents were… a king that is both benevolent and competent and legitimate is relying on the fact that not only did they happen to be born into a position of power but they end up becoming absolutely deserving of said power. History and Crusader Kings II has taught me that no matter how great an empire you are able to build, your descendants will always eventually ruin it in a couple generations.
As for boring, the reason why I wanted them to support the bastard half brother of the old king is because it would have put them against all the evil nobles with whom they are currently working for. It would have caused much more tension because said pretender is half a peasant and that would strain relationships between the kingdom and its allies who see this as a dangerous precedent or a sign of weakness and invade…. Also the party probably would have been playing a much more active role in the campaign if they sided with the Archbishop, rather than just being the lapdogs used on any sort of resistance they are now.