Young players don't magically get better just because they get older. Carlos Gomez comes to mind... Not magic, I’m sure there are swing/mechanical changes but players to find it late in careers. Arcia no where near late in his career. Carlos Gomez had a pretty normal trajectory to his career. Stunk early, was all star level in his late 20's, went back to stinking probably largely due to injuries when he got old. You usually want to wait 1500 PAs before you feel you really know what a player is going to be and even then it isn't perfect. Arcia isn't up to that range just yet and won't be this season. If he hasn't improved by the middle of next season he probably isn't going to do much more improving though.