Thats been a rumor for quite a while. When Guitar Hero and Rock Band blew up I read reports that it increased actual instrument sales by 30%-50% at some music stores. But that's waned since, and I just can't see BB going that route. They have sold instruments on their website for years though along with scooters, fitness equipment, garage accessories, CCTV, and multiple other things they don't have in most stores so ya never know.
I don't think they'll drop their car department anytime soon. They already run with a minimal staff and give it minimal space so it doesn't cost them a bunch to operate it. As long as people continue to go there just because they're Best Buy they'll always have some kind of customer stream.