The first seasonal toe in the water didn't come close to persuading them to go year round off the bat, some routes actually do. The late announcement for 2016 and the fact the route has done OK but not stellar given it is by far the fastest way from NCL to the US and the only non stop service on a hub carrier suggests the market is a little soft for United.
They have only so many B757s and as soon as a better business case for that aircraft's use can be made elsewhere, it's the most mobile of assets, it's gone.
BRS failed as it was too close to LHR, BFS hangs by a thread and subsidy, BHX has plateaued at once daily, twice daily having killed yields. Remember NCL has a number of good options connecting West but only one sensible option going East, that's why Emirates have a true monopoly and United have a seasonal holiday flight. Such a shame IMHO!