I dont think more than half the teams should ever play finals, so I think 9 teams when 18 are playing.
Depends how you look at it.
Look at it this way:
In an 18 team comp, you have the top 9 teams and the bottom 9 teams. With a final 10, the top half all make it, and only the BEST team of the bottom 9 makes it.
If you explain it like that, is sounds more palatable doesn't it?
The NBA for many years had 16 teams making the play-offs in a 27 and 28 team comp. It ceates a lot of excitement among the lower seedings, and it never compromised the integrity of the play-offs because the best teams nearly always progressed to the championship series anyway.
And it will be the same in the AFL. If 10 teams make it, the premiership will still be won by one of the better teams. It always is. But it does allow the season to be more exciting for teams who would otherwise be out of finals action.
So, that being the case, why oppose it?