While your idea ain't bad, you are forgetting that in some cases, a train isn't just a thing to get from point A to B, but rather becomes an identity of the place itself.
Take Deccan queen for instance, the train is synonymous with Mumbai Pune route, and the daily passenger travel that happens on the same. It's rake stays idle for a fair bit of time, but no one in his right state of mind would recommend extending it to Solapur. Similarly bangalore mail is synonymous with banagalore and Chennai, and shifting it anywhere else is not going to go down well with people as some trains have a fan club beyond rail fans.