The only way long haul low cost seems to work is if a full service airline off loads routes on which it can't make a profit to its own subsidiary. It knows the route and the income it can generate, which may not be enough to cover the mainline operating costs, but will allow a profit if the operation is peeled back to the basics with staff on lower pay, no frills and high density seating.
The subsidiary benefits from the parent company's purchasing power when it comes to buying aircraft, fuel, maintenance, insurance and back end support which it would not have as an independent entity. Crews can be seconded and aircraft provided in the event of disruption. Access to the frequent flyer program and connecting flights can be provided within the network.
A stand alone long haul low cost would battle to survive without a backer with deep pockets.