Everything on the Arriva Greenline 757 side is pure speculation until Arriva release details of at least a route change i.e. will no longer serve Luton Airport from 1st May.
You seem to be missing my point. I didn't see the point in stating something that was in the OP with nothing additional when no-one has suggested the opposite.
Does anyone living in the area/use the 757 know where the bulk of the passenger journeys are from/too?
I used to live in the area. The vast majority of passengers outside of peak times are between the Airport and Central London (Victoria and Baker Street). This is why not all the journeys continue to Luton.
I think they've had some success in getting a bit more patronage to the centre - the frequency in the middle of the day used to be every 20 minutes to the Airport with only one an hour continuing to the Airport. I'd be suprised if there was an off-peak service more frequent than every half hour once the airport leaves the route - perhaps even just once an hour.
With the local train service also running 24 hours i'm not sure it'll support running overnight either.