I was just thinking about Paarthurnax, the request of Delphine, the history of Alduin, Paarthurnax and the first dragon war. Is it possible that Paarthurnax was tired of taking orders from Alduin and wanted to find a way for him to take the position at the top. Maybe he didn't think he could beat Alduin on his own (seems likely that he couldn't) so he had to recruit humans to help him. The first war ended in a bit of a stalemate so he just bides his time and lays low waiting until a dragonborn adventurer revealed himself and looked capable of beating Alduin. In the intervening time he comes up with a more final solution to the problem of his brother while he lives on top of a mountain where he can pay lip service to peace among the greybeards whenever they feel the need to hassle him about something.
He's 5000 years old. All he needs to do is stay peaceful during the lifetime of the Dovahkiin and there won't be anyone to stop him taking over where Alduin left off. Well, maybe not the eating the world bit as he seems content with this one, but pretty much doing whatever he wants. This seems slightly more likely after I read his name either means "Ambition Overlord Cruelty" or "Ambition Tyranny Cruelty".