You can learn defense after offense. Offense takes priority. Offense also is what teams pay for.
Oh, also. You look at a guy like Slavin and his defensive contributions. They actually aren't that great. His contributions are actually almost purely offensive.
So, the fact that you mentioned him as one of these defensive stalwarts even though his contributions are mainly offensive, not defensive, just goes to show what the issue with this thought process is. Defensively, he was one of the worse defenders on Hurricanes last season.