That's a very good point about the movies.
It's not the age of the movie that is the issue though as Casablanca is phenomenal for image quality. Just a lot of studios do rubbish transfers. Not wanting to promote another website, but it doesn't gain them anything
http://www.hidefdigest.com review hi def movies, not only on the film but also on the video and audio quality, very helpful as they do sometimes state that the DVD is the same quality so not worth wasting money on the Blu ray version.
Any movie with video quality 3 stars or less is not worth having on Blu ray, likewise the audio.