We don't know what we have so far in JB so it's better to keep him and let him grow. If he's involved in a trade package, it better be for a franchise player like for example Anthony Davis, KAT, etc.
Those kinds of players generally don't move, so JB is pretty close to untouchable unless the right deal comes along. DA is a shrewd GM. He makes winning trades all the time. So if he trades JB it would be for a very high value target.
If we acquire Butler I hope we can do it while keeping Brown, as I believe he would be a good mentor for JB. They have even worked out together in the offseason.