I would probably suggest that the token once-a-day service might come into play, but only once Harbour becomes the least-used station on the line. Currently Bishopstone and Southease still have fewer annual passengers, and Bishopstone gets a half-hourly service, so at the moment it would make more economic sense to close those two down instead
I know the suggestion is partly tounge-in-cheek, but before anybody gets any ideas:
Southease serves several small villages on the far side of the river, and the South Downs Way. The closure of Southease would force the passengers to drive into Lewes, along a very dangerous road into a very congested town with limited parking already. It does get used, and with potential shifts in leisure trends post-covid it's possible it could be utilised more with if it gets weekend trains to/from london.
Bishopstone is useful for avoiding railheading from the western end of Seaford (which would likely go to Lewes if they're getting in the car at all). 1.5km out from Seaford station, at the end of the line, isn't a massive penalty. Also it serves a holiday caravan park which probably makes off-peak flexibility a reasonable factor to consider.
Personally, I'd close Harbour, extend Town's platforms to 12-car and build a south entrance. It's approx 600m from the entrance of harbour to the entrance of town, cut that down to 350m and it's added very little time. It might be feasible then at town to put a mid-platform signal to let shorter trains stop without closing the crossing first, saving on congestion. Peak-time trains from Seaford could then be 12-Car if required. Off-peak weekdays you'd continue to run the present half-hourly service to brighton, but on weekends you could divert the Victoria train that terminates at Eastbourne to Seaford (leaving the Ore train to continue serving Eastbourne) and give both Branches 1tph all-day. I'd also put the old bishopstone beach/tide mills platform back as a weekends-only request stop. How you fund (and get permission for) most of this is a very different question...
Crew & Unit diagrams would also probably be rather dramatically affected by this. This would allow the trackwork at Newhaven to be slightly reconfigured for access to the Marine freight terminal, possibly making maintenance savings depending on how much the level crossing drives up the costs.
Back in reality, you may as well keep serving harbour with 2tph as long as it's open given it makes no difference to the times, and you'll still have to pay for the upkeep. Best the branch can hope for is for a diversion of 1 of the Victoria-Eastbourne trains on a Saturday to restore all-day direct trains. Might not make too much difference but could be enough for people with luggage to use the ferry as no change, and for (mainly family) day-trippers to use Seaford instead of a rammed-out Brighton.
