I think Casablanca is similar to Citizen Kane in that it was amazing compared to everything else at the time, but many movies have since done their shtick better.
I look at it like this, everyone thinks the Dark Knight is the greatest movie ever right now, but 80 years from now when every film has a well acted villain by default, people will wonder why the hell anybody cared about Heath Ledger's Joker.
Tbh I don't think Citizen Kane has been surpassed. It's certainly overrated with every other person parroting it's the best film ever - but it is one of the best films.
Timeless classic for me.
Casablanca is a film I enjoyed but can live without. Kane is a different thing.