Rockbox was created because stock DAP firmwares suck, and people like to hack. Native music player apps for android suck, they really suck, they suck really hard. I would consider it an evolution of rockbox, not a reinvention.
My sons and I have been using Rockbox for 10+ years, on many different devices. We intuitively understand how it works from a user interface perspective, even as we upgrade devices. Native Android apps don't work the same way, with file/folder access, automatic playlists, and bookmarks.
I'd love to see Rockbox on Android grow into an app with a native Android UI that works the way Rockbox always has, regardless of the playback engine. Skinning would be a plus, but honestly Rockbox is still my player of choice in Android with the default build and no skin loaded (could never find Cabbie V2 for 1920x1080 resolution).
Thanks to the developer community for everyone's hard work!