Boston is typically in the top 3 in North America) so you're going to have a lot of business traffic filling up the business and first class cabins. Coupled with cargo, you could very well make a profit flying a route at 40-60% load factor and I am sure there are plenty of carriers who do so.
AFAIK, that's how UA justifies it's BOS-LHR route despite having minimal domestic feed - they have corporate clients who a) have huge airline specific contracts, and b) predominantly fly in premium cabins regardless of cost, so they know they'll make money regardless of how.many plebes fill the back of the plane. Pre-Covid, Apple alone bought 50 business class seats between SFO-PVG every day
- at the time, Uniteds entire J capacity on that route was 168 seats, and Apple accounted for 7%
of all UAL revenue in 2019. You account for enough revenue, they'll make something happen for you.
The other half of it being Cargo: you're probably right. AFAIK, AA's biggest cargo station isn't actually stateside - it's Heathrow. It's how AA justifies having a flight from RDU to LHR when their major CLT hub is right there - pharma companies are willing to pay lots of money for well controlled cargo conditions. Per friends who work for AA, that flight is ideal for non-revving due to the fact that so little demand connects through RDU compared to CLT.
However... I'm not sure how this really helps JetBlue. On the Cargo front they're flying an a321LR- the LR belly tank(s) take up 2 of the 10 LD3-45 slots in of themselves, and my understanding is that they have functionally Zero cargo capability once you account for passenger baggage
, which isn't the case on the widebody competition. On the premium corporate passenger front, while Mint is indutibly the best premium seat domestically, it's a much more level playing field when flying intercontinental - and they're new enough to the game that I don't know that they have the corporate game when every legacy is also on the route. JetBlue also isn't a member of a global alliance, nor do they have a strong codeshare network in Europe, so it's likely really only Americans on their flights. Plus, the 321 is solidly 25, 30, minutes slower on the crossing than the wide-bodies?
I get that JetBlue is positioning itself to grow into being the next full service carrier - NYC/BOS-LON is basically the golden goose. Perhaps JetBlue is willing to stomach losses for now, but I feel like they're going to need to find some other routes where they're not playing against legacies with corporate ties, such as more leisure focused cities where they can leverage their US feed to serve airline agnostic tourists wanting to go to say, Barcelona or Venice.