Whilst I agree that is was a legitimate mark, I am not sure that it will always be the case. The
AFL rules already state
"FREE KICKS
15.1 INTERPRETATION
15.1.1
Spirit and Intention of Awarding Free Kicks
It is the spirit and intention of these Laws that a Free Kick shall
be awarded to:
(a)
ensure that a Match is played in a fair manner;
(b)
provide to a Player, who makes obtaining possession of the
football their sole objective, every opportunity to obtain
possession;
(c)
protect Players from sustaining injury; and
(d)
a Player who executes a Correct Tackle which results in an
opponent failing to dispose of the football in accordance
with these Laws."
Whilst we do want an exciting game to watch, protecting players from injury is already stated as being important. It won't surprise me if there is a move to ban the "flying knee".