The fertility rate in Australia is declining and immigration levels
are not yet set at levels that might lead to population growth. Might
this influence the revenue and earnings that Quantas Airlines could
achieve in the future?

I don't think so, because the main business for Quantas is to
transport TOURISTS. Australia ia an extremely touristic Country...:)
So, it doesn't matter if they decline in population, they still have
their attractions, and tourists will buy Quantas tickets as usual. I
think they even will begin an agressive campaign to atract more
tourists to Australia.