To say that his sacrifice is idiotic is silly. For one, by even joining in the battle at all, there was always a very good chance that Ferelden would be without Alistair to take the throne anyway, so it's not like he's so super precious post-Blight that Ferelden would be doomed without him. Being a Grey Warden, his duty to combating the Blight supersedes that of his obligations to the throne post-war, so if he takes it upon himself to kill the archdemon, then he is fulfilling his purpose just fine, even if the Warden doesn't like it. There's also the fact that Anora would be a suitable ruler on her own, unless the Warden turns against her.