I understand the powerful incentive to rinse every last penny from airport express services. Indeed, the contract to run the Southern franchise was awarded on the basis that GX would be a premium service attracting premium fares, so they have to try and run it like that.
The fact people have a general sense they'll be ripped off if they take public transport to the airport, however, and that it will be a pain to work out how to avoid that, surely contributes to the enormous traffic congestion around airports? And it says to visitors "Welcome to Britain. Get used to being ripped off." Particularly when, as with GX, they've done little more than paint some of the trains a different colour to justify the enormous fare premium, which becomes apparent when you travel. Most European airports don't do this.
Getting rid of the premium rather than finding sneaky ways to increase it would certainly make things a great deal more straightforward for regular, non-airport travellers too!