The only way around that will be to stop allowing the Ridacard to be accepted on the Airlink and this would push people onto the X18. For fare payers, the X38 is also an option.
Ridacards are used by a lot of airport staff, plane crew and residents who hold one going to the airport. Taking that away would undoubtedly push them onto the competition.
In my experience the additional stops barely makes any difference to journey time, often arriving at either end a few seconds ahead or behind of the orange buses.
I have heard a number of drivers say the Air Link should be set down only going into town, with some supervisors agreeing as well. Delays caused by west Edinburgh residents hopping onto that instead of one of the city buses to save a minute or two in short journeys are commonplace. I would even go so far to say some of these residents would flatly refuse to use anything other than a 100.
There are some that will allow an X18, 31 and 26 to go past them only to be met with a bus that's completely full from the airport. I can see why it's an attractive option to use exclusively over the other services with the frequency and quicker journey times though.
The intermediate fare and stop also subsidise the service during lulls in airport demand whereas Bright Bus have to (almost) rely entirely on airport journeys. Just a few intermediate fares can cover the cost of wages/diesel.