I reckon this comes down to Grigg himself, not the coaches. It seemed to me he just thought it would all come this year, without realising he still had to work hard and improve his game.
Every single report coming out of pre-season had Grigg as being the standout. Players, coaches, footy department staff...everyone was raving.
He then started the season as a half forward, a position I've never seen or heard of him playing before, and he was quiet and eventually dropped. He's oscillated between SANFL and the best since.
If I had to choose what the cause was, I'm not putting it on Grigg. I don't know of course, but I don't reckon your position is supported by the evidence.