The biggest problem I can think of with implementing the old pick/ban music is that the new Draft format doesn't have just one piece of music. It's got a modal system that slightly adjusts what you're hearing as you cycle through the draft, so you hear a different piece during champ declaration, a different piece during bans, and then a different piece during the picks. It's not as easy as just slapping the old theme into ranked and calling it a day.
That being said, I 100% agree that Riot should bring the old theme back in one way or another. Might require more technical work, but it was a fantastic piece of music.