The reason why the music player was removed, is because it was a prebuilt application that only used the launcher as ‘control unit’. It is currently the same situation with the fitness application.
This is something that i personally don’t like (forced external dependencies), that’s why the step counter and weather were remade as a launcher built-in.
The philosphy here is: either have it built into the launcher or only have a external app.
I might build a replacement for the music player as soon as all the current bugs on the launcher are ironed out.