One thing that has been missed in the comments about how many people live near the stations is that a journey has two ends. It is no use having lots of people living around the stations if none of the places they want to travel to are convenient to the stations. Places that residents want to travel to are likely to be the main shopping areas, of which only only Shanklin is not too far from the station, although it is not a massive centre. There are the large supermarkets, which are not near the railway, the various council offices, which are mostly in Newport, and sports, etc. facilities used by residents, which are again not near the railway. This leaves internal journeys by locals as a series of penny packets: going to see a friend who also lives by the railway, going to the Pier Head to do some sea angling, a darts match in the railway inn, etc. The main use of the line for the majority of passengers is connecting to the Cats and that is now such a hassle that people are likely to go by car direct to the Pier Head or, as they have to change anyway, take the Southern Vectis bus. And of course you could buy a through rail ticket from the island to the mainland from the guard; I understand that you can’t buy a ticket on the RRB.
Tourists are not going to use the line much during their holiday as other than the Steam Railway and Ryde seafront, it doesn’t really go anywhere on the main tourist map. If they weren’t put off when making their arrangements and did use it to get to and from their base, they probably won’t have got a particularly good impression of the railway.
I doubt that, in total, the closure has cut the number of visitors to the island as such is the market now that there is always someone else to grab the booking. Maybe there was a small loss (without replacement) of day trippers who reckon that it will take too long to get to and from where they want and go somewhere else. In my opinion, the damage is that a lot of those who would have arrived by rail have chosen other modes, so reducing the island’s revenue in the period when it is usually at its highest. And that some of those who did travel are going to be put off and not come by rail again.
EDIT: In some ways the perceived difficulties and delays of using the Cats are probably a greater disincentive to using the line, particularly for day trippers.