The system now maintains consistent flight number when creating routes with "Create route like this next day". So this proposed enhancement is not needed for any of the new routes, and will not be needed in the future. As far as converting older routes created before this feature was implemented, it is optional, if the player wants less crowded look.
I don't think this is such a high priority that Sami should be programming it. I went through all my routes, when the collapsable routes were first implemented, and I got it done in about an hour with the tools already in place.