Fare revenue actually goes to the government not to the operator - and the cost of funding the bus service is outside the rail franchise so they don't get the costs back
When Parkway opened in 1999 the shuttle bus was free and remained free for a number of years until the Bendy buses were introduced. Now I can't remember who operated those free coaches but I think it was the airport rather than the rail operator. The service is no different than the car hire and remote car park buses. Seeing that it is the airport that has applied for an environmental assessment it is the airport that is looking for a replacement for the buses rather than the rail operator.
On that premise there is nothing stopping the airport bringing the link in house again and providing their own bendy bus service. Now whether they would want to do that and absorb the cost when they charge for everything else including plastic bags is another matter but then that brings back the question if they provide a dedicated link at say a cost of £100 million would they then want to run it for free. If they didn't then there is the same set of passengers that is queuing today to pay the fare for a bus that would queue to pay for some sort of rail link so where is the integartion and seamless travel? The link will only improve if passengers just get on it and then go without dipping their hands in their pockets.