Before everyone gets on their high horse against the prosecutors and Freo, there were actually two sets of charges against Yarran. There was the stabbing and police chase which I think he pled guilty to and served 6 months. Given he only got out of gaol late last year, I reckon that would have put a pretty big dent in his footy career.
Then there was the assault of the family member, which I think this is referring to. The only witnesses to the assault would likely have been family members, including the injured party. Getting a conviction in these circumstances is very tricky, but that that doesn't mean the DPP should not lay the chargers.
There was also the move on notice in Leederville, which is neither here nor there, but perhaps speaks to a pattern.
It's possible that Yarran is, in a broad societal sense, a victim of circumstance; but as a mature-age pick-up at a footy club that only played a few games you don't get too many chances, and Yarran had exhausted his.