Not only do take-offs spend large amounts of fuel; the fact of a landing and the servicing and refueling in the airport also cost the airlines.

Not having to land but instead having to carry some extra fuel while the engines work in their most efficient mode, and the air drag is low at a high altitude, should make airlines money. Else they won't try that :)

It's basically the business model for some airlines like the Icelandic and middle eastern ones. Hub in the middle so you're not paying (potentially extra) staff to rest on an aircraft and/or hotels at each end. Be at home more rather than an out-station. Fuel up more where its cheaper. Do some "scissor" routing. Fly cheaper/less specialized aircraft. Not as easy for Qantas to get all those advantages. Cargo airlines do the same to carry more cargo.

At some crossover point, the reduced fuel carrying (and max capacity requirement) makes up for the additional takeoff+landing. When we're talking ultra-long-haul, constantly fighting against more gravity due to incr fuel weight adds up to a lot of cost/burn.