Its not just Cairns, be a million or two including Townsville plus outlying areas, many would do round trip in out of capital cities like CX currently do.
A million or two might be a tad optimistic. According to last estimates of population by LGA it's about 550,000 stretching up to almost 700,000 if you include Mackay and Mt Isa in the catchment.
Having said that, tourist hot spots, such as coughet and Krabi sustain long haul services, with smaller and/or less affluent populations (although they are much closer to Europe)