Besides the fact that Bruce's training has been done in the films (Batman Begins), animated series (Batman - The Animated Series), animated movies (Batman - Gotham Knight), video games (Batman - Arkham Origins "Initiation" DLC), and ad nauseum in the comics themselves. Seriously, we don't need to see more of Bruce training to be Batman.
How many more times do we need to see a Batman origin story? At least this has some element of originality by focusing on Gordon and early versions of the super criminals who will become Batman's rogues gallery.
Furthermore, if the show is successful and lasts a few years, I can see them eventually doing a time jump and skipping several years to Bruce being 18, leaving Gotham, and beginning his training, occasionally showing that while Gordon still attempts to clean up Gotham, and maybe him and his wife have Barbara.