Simply "Would it be profitable?" isn't enough of an analysis. Let's assume for purposes of this discussion a NCL-JFK (or wherever) long haul route would indeed be profitable.

The question is, "What is the MOST profitable way to deploy these limited assets (aircraft, crew, etc.)?" Sure, NCL might be "profitable" but apparently not as profitable as other options.

And don't forget the network effect. The "Why doesn't BA fly more long haul from the regions" question comes up regularly here. Each non-LHR longhaul flight would take away connecting passengers from the LHR hub and in turn affect the profitability of those long haul flights, which in turn could lead to service reductions. BA has chosen to protect its LHR hub as much as possible, which makes sense IMO.

Yes, I know; LHR is a crowded hot mess so fewer passengers passing through could improve the experience for the remaining ones. But that is not what drives airline route decisions unless it is extreme enough.