I really dislike this outcome driven decision making, it takes away common sense approaches.
Defenders have to anticipate
a) jumping (Thompstone)
b) ducking (Luamanu)
c) team mate successfully tackling (Hartley)
d) flying (Bigger/Russell)
Attacking players have to
a) take care that defenders are not in a crap tackle position, and
b) if they are, carry the ball in 2 hands and run a bit slower
No there should not have been a swinging arm, but had it connected mid back would JG have given it a second glance? Doubt it very much. That it connected to the head was solely because Wood completed his tackle half a second too early. The replays were all slow mo which make it look bad, real time I didn't even notice, it was that fast.
Rugby absolutely doesn't need serious and deliberate foul play, but the onus cannot be 100% on outcome, because so many incidents are not dependent on just singular illegal actions, rather a series of unfortunate events that happen so quickly in a sequence such that someone can't react quick enough and someone else gets hurt.