That's no different from travelling in from west of Worthing and needing a ticket to get to the arbitrary line at Worthing.
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...
The more I think about it then if this is about cost cutting I think any bus/train travel west of Portslade, north of Preston Park and east of Lewes may be vulnerable to cut back.
My random guess is they will at least look at reducing the size of the rail travel zone to being just between Brighton ( Including Preston Park and Hove ) and Lewes.