The S61's were no doubt expensive to operate, but the operation was perhaps subsidised by the airlines that benefitted from the interlining of high yield business and first class passengers, so although losing money, not at such a rate as might have been. Before the M25 there was really no easy road link, and then when it was built it became like a magnet that caused it to almost mimic the previous A and B road options. I wonder if today it would be quicker to take the Paddington Express, tube and then Gatwick Express between the two airports than by car service ?