Wasn't a mistake.- Umpire makes a mistake (failing to call play on)
Didn't need an excuse.- Umpire makes BS excuse (natural arc)
So they should have.- AFL supports umpire
As they did in the week itself, the previous week, the week before that, et cetera.- Following week, other players apparently (I haven't seen it for myself) also run off their line.
Do you really think the umpires are now going to change their tune and call play on, after defending this new "natural arc" ruling? Do you really think they're going to want to open that can of worms, and give people an excuse to ask more questions on the issue? Of course they wont.
Do you really think that if any player was called to play on in similar circumstances at any point this year we wouldn't've seen that footage by now? Do you really think that players decided to take up drifting to their favoured side based on last weeks use of the phrase "natural arc"?
In short, the fact that they've failed to contradict themselves is not in any way evidence they were right from the beginning. If anything, it's to be expected.
In short, you've tangled yourself up in a mess of speculation, misinterpretation, sour grapes and wishful thinking. I hope you can come out the other side better for the experience.