I guess it will leave a hole if they don't carry it on. My wife flew there and back on Tuesday taking the second flight to Gatwick and back on the penultimate one. She said both the early flights were fairly full and mentions from fellow passengers that it was so much more convenient than the train.
Smart move would be to carry on the extra flights for as long as the demand is there. There could be quite a few converts to flying rather than 5 hours on the train.
Might help to market them a bit too. Not seen much promotion of the extra flights.