Freddy Laker always said that the big network airlines (he had other words for them) ganged up and put him out of business.
But what really happened is they put up with him while he was small, but once they started to lose business they matched his pricing. (Well one did, and the rest matched the first) Given the choice of an elderly DC10 with a stop in Bangor, vs direct flights, or with connectivity vs point-to-point, it didn't take long until he was flying less than full.
Norwegian has addressed one issue by using new B787s and opening new markets. But can they generate enough profits (not just revenue) to sustain the capital costs of a new fleet. Summer and Winter? Only time will tell.