The stats I'm talking about are the historical data for goals vs behinds. Whilst it's not a flawless model due to factors already discussed, it displays a pretty clear trend that goals are continuing to be scored at a higher rate than behinds, and a natural link to that is accuracy of goal kicking.What stats don't lie? There are no stats earlier than 2000 on set shots.
I must admit though, I did simply assume it is worse.
The figure in 2000 for set shot conversion was 58.9%. By 2015 it had increased to 62.3%. So it improved from 2000 to 2015. However in 2000, 49 per cent of kicks for goal were from set shots whilst in 2015 it had come down to only 45.7%.
So let's put that into perspective...
Hypothetically, if a team had 20 shots at goal on a day in 2000 they would have had 9.8 set shots for the day, and converted 5.7 of them. So they missed 4.1 set shots.
In 2015, they'd have only had 9.1 set shots for the day and converted 5.6 of them. They'd have missed 3.5 of their set shots.
So although the 3.4% increase in set shot conversion seems quite high, in practical terms, it's less than 1 miss better. Better, but not much better at all.
Those stats from 2000 - 2015 would also support the view that goal kicking is actually improving rather than going backwards. You didn't actually specify in the OP that this discussion was for set shots only (as opposed to also including 'in-play' shots on goal) but regardless, that 3.4% increase between 2000 and 2015 stands no matter the frequency of set shots vs others.