You can't say the last call cost Cas the match, because that then negates all other wrong calls prior to that, to being less important. But this is a pure lack of understanding with regards to temporal theory.
If you look at the last call of the match and say well if that had not been given, Saints would have lost.
Then Saints fans can say well what about the ball steal earlier in the match or the ripped balls from Walmsley. You then respond by saying well that was earlier on in the match, as if the rest of the match would have followed in the same fashion still lead to the Turner incident with the same points on the board.
But time does not work like that, if Manu gets the ball steal or Walmsley gets a penalty for 2 on 1 tackles. Then the Turner incident even never occurs, you follow a different temporal path.
So the last call is not the crucial call, it's just another call which can lead to 1000's of multiple outcomes. Even if the Turner call would have been given to Cas as a knock on, there was over 2 minutes. This would have been enough time for 2 sets of 6.
People just don't understand time lines and temporal physics, this is why they place far too much importance on the last thing to occur and deem all other things prior to this as some what less, when in fact all decisions have equal weighting and equal unknown outcomes. You can't take one call in isolation. If one call is up for grabs so are the rest.