I thought AC had canceled their planned LAX-SYD service (replaced with codeshare on UA). This route has made the list year after year of the highest yield route in the world -- so it has been attractive for a lot of airlines. Even AA had plans to operate LAX-SYD many years ago (prior to Oneworld). The high operating costs along with govt restrictions have served to keep some out, despite the profits to be made. DL is planning to cut up to 20% of domestic US capacity so it is choosing new international routes that are high profit and which it knows can work. Given their situation a lot of thought has gone into these expansion plans and route selection. I don't think they plan to try it for a few months and see what happens; rather it is part of a long-term strategy. If they need more capacity quickly, they can swing a dozen 744s (from NWA) onto the route, if need be -- very few other airlines in the world, much less the US, can do that. Of course their current route authority is for 7 flights per week, but that can change quickly.
As for feed at LAX, DL has decent feed of their own (from SLC and CVG) which actually gives excellent coverage to all of the US, albeit with an extra stop. But all of the Skyteam options are worse, so for Skyteam loyalists this counts as a convenient connection; and as pointed out, Oneworld doesn't do much better for much of the US, requiring connections through ORD or DFW instead.