Very interesting point on the business class uptake EI-BUD. I did wonder how many of those seats leave empty.
In total agreement on the capability of the 757. The admission I talk about is exactly what you said, that it can be used more profitably elsewhere. Eg 3 or 4 hub to hub sectors per day internally within the USA. Couple this with the shift of a large amount of Atlantic flying transitioning to B767-300 out of EWR, its certainly seems United would prefer to use the 75's elsewhere.
An interesting point on the 321neo is that United have shown no interest whatsoever. Infact probably the opposite having ordered 100 737MAX9's which by all accounts so far don't have the capability to replace 757's across the Atlantic.
I suppose in the longer term the writing is on the wall for this service if it stays as is. In reality this £9m is just a stop gap while BFS and Stormont try to beg someone else to take on the route when United scarper.