I don't think Greenline are going anywhere and this move by National Express is just really to introduce some more competition on the route...
Greenline's website states a new bus timetable, which only came in effect on the 24th March, so seems like they are here to stay. Download the PDF from the below link:
http://www.greenline.co.uk/uploadedf...313_010213.pdf
I would imagine the Airport will possibly market the National Express option to London as their preferred bus operator on the route and give them the convenient bus stops outside of the terminal. If they are people that wanted NX to come to LTN.
When the press release states buses up to every 10 mins at Peak Times and up to 150 buses a day. I wonder if these will be dedicated buses to this route, or if they will just divert more buses to and from the Midlands and North travelling up and down the M1 to call at the airport?
If you have ever been to Milton Keynes Coachway, which is a major National Express stop at Junction 14 of the M1, I would imagine they have a similar number of buses calling there each day and with similar frequencies. None of these are dedicated MK to London services, and just buses stopping enroute as they go North and South on the M1 to their final destinations.