Well, it isn't true that just because the flight's full it's turning a profit by any means, there are plenty of routes that operate at 90% loads and lose money, and others that cannot (on current passenger mix) ever make a profit.
Also, a route like LHR-NCL is fairly well down the priority order for fleet allocation and often gets what is left over, rather than a deliberate policy to fit 757 capacity to the route.
Incidentally, I'm sure that passengers are delighted that the EI monopoly on Scotland-DUB has been broken and I'm not defending a high fare single operator by any means. But it's a slightly false position at the moment because FR and GO are in a private spat on a lot of the routes, the market is not sustainable at those levels in the long term.
Nevertheless, the point I was making is that it is not inevitable the full frills carriers will get duffed up by the low cost boys providing they get their act together. Some lowering of overhead is required of course (and this is not intended as a green light for certain people to blame everything on pilots) but the likes of GB Airways have shown that it is possible to remain successful even with such heavy competition.