Apologies for the long Post
Time will tell on all of this, so I cannot speak with certainty. I'd like to preffice this by saying League is my number 1 sport, Parra my most beloved team by a long way, however, I also love football (go the Reds!).
I dont think League has anything MAJOR to worry about for a number of reasons.
Firstly, Football is not a contact sport like League, Union, AFL, NFL or even Ice Hockey, so the unique thrill we get from a big tackle or a line-break cannot be replaced like-for-like with football (ditto when playing it recreationally)
Secondly, NSW and QLD, but Sydney especially is used to having 3-4 sports competing for the corporate dollar. NRL, AFL and ARU are entrenched in Sydney and split the merchandising, sponsorship and support up between them. We co-exist in relative harmony, with some NRL fans watching AFL and vice versa (With League dominating).
Thirdly, the A-League got off to the right start by having one-city clubs with no history, and there is the potential that a tribal support will be formed nation-wide. Sydney FC managed this in their first season, but so-too did the Northern Spirit and they died pretty rapidly after a good opening season.
Tribal support is something your parents pass-down to you, it is long lasting - almost like a religion! Though I was happy when Sydney FC won the A-League, I think I would burst into tears of joy if Parra ever wins a Premiership - such is the difference.
Therefore, I see Football taking an increasing share of the supporters market in Australia, but it will not kill League or relegate it to number 2 in NSW. If football takes 30% of the existing support away from NRL, ARU and AFL, each code will lose only a percentage of that.
I believe Rugby Union has MUCH more to lose in NSW (and Australia) because they make so much of the international nature of the sport. AFL also has A LOT more ot lose nation-wide because outside of NSW and QLD they have closed-shops - 90% of them follow AFL. Therefore, if 30% of AFL fans flock to football they will see a much larger percentage drop in their support.
If the A-League can act as a breeding ground for 17-21 year old stars on their way to Europe, as well as provide a retirement spot for Australian football legends like Kewell and Viduka, all sports in Australia have something to worry about. Participation rates are already massive with football and even a small translation of that into passionate support will affect other codes.
I think League will live-on though, and that both can exist.