If a passenger wants to fly from Newcastle via a much larger airport for an onward long haul connection, how would Manchester compete against the existing hubs (with much greater frequency long haul) of Dublin, Heathrow, Brussels, Amsterdam, Copenhagen, Paris and Dubai ? If there was an airline with a sizeable hub at MAN then maybe but when long haul at MAN is a bunch of airlines in different alliances and each with their own agenda...
Furthermore, apart from those specifically connecting to another flight, the train probably has far too good a frequency for a flight to compete - meaning that Flybe would likely really struggle on revenue given that they see so little cash from those connecting to long haul flights
EK777 - I don't mean to be nasty, and it's gratifying to see someone so keen and coming up with ideas, but I just don't see a NCL-MAN route happening. If on the other hand you could persuade LH that there is a case for NCL-FRA...