That's no different from travelling in from west of Worthing and needing a ticket to get to the arbitrary line at Worthing.Practically would be a nightmare to police wouldn’t it ?………I’d buy a train ticket from my local station (no barriers) to Preston Park but stay on the train into Brighton ? Can’t see them doing ticket checks within that 10 minute stretch. Whilst this may already be happening for people travelling in from outside the current travel inclusive zone the numbers would jump massively if the zone was reduced that much.
Shifting the line between free and charged rail fare isn't an impossible issue, although realistically it will always need to be free on the Brighton-Falmer-Lewes line.
I'm not saying this will happen. It depends how much money needs to be saved, and also how "sacred" the current travel zone boundaries are to the decision makers.