Unfortunately, they painted themselves into the corner from the time they selected the design. The small station in the Airport precludes any use of large trains. Small trains that can fit into the airport station would be totally swarmed at the in-between stations, if the service operated at the regular GO fare. Hence they have to keep the fare somewhat higher than the regular GO, in order to prevent overcrowding. But the higher fare kills off-peak ridership, resulting in the need of a large subsidy.
If they raise the fare, that might not divert business travellers who can expense the cost, but the overall ridership will go down. It isn't even guaranteed that they will collect more revenue as a result; could actually be less revenue.
Not even sure how they can enhance the service to improve the usage and/or improve the cost recovery ratio; short of a very expensive and annoying rebuild of the Airport terminus.