The reason Apple keeps Logic is they generally have a strategy of trickling down features from big pro apps to smaller consumer apps. Logic -> GarageBand, Final Cut -> iMovie, Xcode -> Swift Playgrounds.

The AVID idea doesn’t make much sense as Logic and Final Cut are already integrated into their product lines. I think there’s a certain amount of truth to the brand prestige argument, but I think the more important point is that these capabilities are actually core to Apple's product strategy, they just manifest themselves in unintuitive ways. E.g., Apple's AUv3 strategy would never have worked without GarageBand and GarageBand is Logic.