You don't believe they are, based on your opinion, rather than any facts. Thankfully there's still a minority of us on this website that actually work in, and have acquaintances in this industry.
Norwegian have an extensive connection network from LGW, using both their own metal and easyJet, and the 787 is significantly more fuel efficient than the majority of aircraft on the LON-NYC/BOS route. They also have brand recognition on both sides of the Atlantic, unlike jetblue, who will be relying on non-affiliated, largely leisure orientated passengers from the USA.
If you think that Norwegian, a company who already fly over 1000 passengers in each direction every day on the LGW-JFK route, have significant brand recognition and a lower (and reducing) cost base than any of the competition are suddenly going to become the "weakest player" the day jetBlue announce (or don't) flights to any London airport, then you're stupid.