Some observations are valid, some not so applicable. If the fixed cost relates to a set number of rotations of profitable routes it makes sense to amortise that cost base, on the other hand, if additional non-profitable rotations are added it may result in additional staffing and overheads and thus the previously stated "fixed costs" are now increased dragging down the overall group profitability. More frequent schedules on popular routes does not automatically lead to more profit as this model demands high pax numbers/ freight or combination of both. Having said that, even with full pax and headwinds a 787 can easily cope with the routes Norwegian were operating and still allow a modest amount of cargo, In normal weather conditions I've never seen a 777 or 787 with no underload. The issue appears to have been the lack of a proper load optimising system, communications and bad cargo contracts. If cargo timing is not critical then eventually it all gets shipped.
Getting back to the LC LH model, while some routes may have been revenue positive, taking the whole operation it was a financial disaster, bit like running a supermarket with one profit making product, eventually the overheads will kill the company. Some posts elude to the ticket pricing, frankly, compared to the majors it's not cheap and the most successful routes are those on which they created demand rather than try and poach customers from others.
You cannot borrow your way out of debt, this is what is being attempted now.