Marine stopped being served in something like 2008, the Ferry terminal moved long before that. Marine was only shut to passengers because the building was falling apart after years of no maintenance.
NR and FOCs are actively looking at introducing additional aggregates paths. However, I agree with you that Southern trains should stop at least once per hour. I thought in the normal timetable pre-covid they did?
The station serves the people who live there travelling to Brighton, Lewes and London. Newhaven Harbour was getting c.50k annual passengers a year pre-covid. Not massive, but still people who would be disadvantaged. Given the advantages of closing the station are next to none (you save yourself 30 seconds runtime to a dead-end branch, and the cost of upkeep of 2 platforms and a ticket machine) why encounter the disadvantages of the closure procedure?
I have not been able to find any timetables online pre - covid that show Southern stopping at Newhaven Harbour. I know that they did back in about 2009/2010, as I got on a train from there to travel back up to Martins Heron where I was living at the time, having visited my parents over the weekend who live in nearby Peacehaven. (Please don't get me started on the fact that Peacehaven should have a railway station!).
The ferry terminal from memory I believe moved back in the 1990's, due to the decaying structure of the old building. The current ferry building was initially built for car/lorry passengers, but due to the state of the foot passenger terminal it was decided that it would be cheaper and better that you combine all passengers in one terminal building.
I don't disagree with your comment about the fact that Newhaven Harbour does serve that area of the community, but like the fact that those that actually live in Newhaven have to walk over the swing bridge and down to the station which is I would say a greater distance, then those having to walk from Newhaven Harbour station to Newhaven Town station to catch a train, I can see that there might not be a need to have the harbour station where it currently is located. Hence, why I think it may get closed. If the plans for a new ferry complex get over the ground and things start to get built, then I can see Newhaven Harbour station being more towards where the remains of Tide Mills station exist today. With regards to people travelling from either Newhaven town or Newhaven harbour travelling to London, that is only possibly via Lewes even before Covid. There is no longer any direct London trains from Seaford.
Most people I know from the East key area of Newhaven, that do work in London actually walk down to the bus stop by Newhaven Town station to get the bus to Brighton and then the train to London as it is cheaper.