Why would airport pax be concerned if the train then continues to Brighton - doesn't this make the best use of existing, heavily congested, tracks?
Yes, but the biggest problem is that when that train rolls back into Gatwick from Brighton at peak times, it'll be full of Brighton passengers and there'll be no room for the Gatwick passengers. As one who's used this line many many times, when its busy its a misery and it'll not make travelling FROM Gatwick into London very pleasant at all.
Andy