In general, that is probably correct. But that "traditional wisdom" has always assumed that every business pax is like every other business pax on the NYC-LON route(s). The reality is that pax have business destinations all over and around London. I suspect that a pax that has business with Nestle in Croydon would much prefer to fly into LGW than LHR. Similarly, when I need to get to Brighton, I def would not want to fly into LHR. DL doesn't have to get all of the NYC-LON traffic to be willing to fly into LGW for the route to work, or even a majority of that traffic - they just have to get a share.