I don't think it's makes much of a difference to the standard of the senior competition whether the thirds is Under 17 or Under 18.
While some kids might not be as well prepared for open age football that year earlier there they kids that are likely to play Reserves not seniors. So as for the Heathcote League not being able to step forward until it goes to Under 18's would have to be classed as another wayward comment from you young Nicko.
You blokes take this stuff way too seriously. The point I am trying to make and I would say it for all leagues, is to get the most success at senior level in a competition, you need to make your highest under-age level as good as it can be.
While the U17s system works OK in the HDFL, serious consideration should be looked at to raise the age level. It may take two, possibly three years to see the full benefits, but they will come.
As for the Mounts v Riddell argument, I saw Pleasant on three occasions this year, Riddell three or four. I saw both grand finals, both against good opposition and they are both good sides. In my opinion, Riddell were a better side. Nothing against Mounts though, they were brilliant.