V Australia only has a few flights a week so far and Delta is five times per week, so neither would be eating into UA or QF's markets too much. Plus, the market is not a finite size - the new competition and the GFC has meant lower airfares which in turn means new markets have been opened up to passengers who may not previously have been able to afford to fly.
A personal experience I can relate - I went to the US in early July on a work trip, and arranged for my family to join me in LA at the end of that trip for 10 days in the US. For four of them, it cost me about $4500 return on Delta (and would've been cheaper on VA had I been quick enough), about half the usual fare. There's no way we could have afforded to do it if there weren't heavy discounts on the route.
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