Let's be honest with ourselves, crowds as a whole have been declining for a couple of years now. Last year's were certainly down as proven by the RLFans attendance sadist.
I don't think the WC or the format changes actually have anything to do with it. A lot of clubs have all seen huge spikes in attendance in a short space of time and it's no surprise that they have subsequently fallen. Think about it, Hull, Wakey, Wigan, Saints, Leeds, Wire, Huddersfield even Leeds all saw crowds go up massively, doubling in some cases. In 2005 I was at a Leeds-Widnes game with over 17,000 at it, they rarely get that for anybody now.
Also there has been a SL fatigue that has set in especially as the much desired levelling out of the competition hasn't really happened. With the top 8 as well, the early rounds seem to matter less and less.
It's not all doom and gloom though and we're still historically high. The problem is if people start seeing it as a crisis and as proof of the sports decline, which let's face it will inevitably happen.