I believe AA has had a longstanding contract with GlaxoSmithKline to operate the route. Even still, the Triangle has a good concentration of high tech and biotech companies and offices - these business have a higher propensity to travel than say what you have across the state in Charlotte where a number of banks have their back offices and low level functions housed.Well... It helps to have a partner hub on a end, ie BA in London for AA and Delta in Boston for Virgin respectively. Honestly, I think this London flight is going to be touch and go; I think there's too much competition on the route, too little feed. AA feeds their own and BA, DL feeds theirs and Virgin, JetBlue hubs here. The only visible competitive advantage / differentiating factor is that this is the only Star option.
That said, if AA can justify flying to London from Raleigh/Durham and the research triangle (though I'm given to understand that particular flight is really about cargo) UA should be able to justify Boston, but if I were UA management I would like to see load factors after about 3 months.
It would be nice to have as many carriers as possible on a route, but I cannot see United making a serious go of the route. They would be joining Delta, America, British Airways and Virgin Atlantic, plus JetBlue launching in 2022.