Wiki (
https://en.wikipedia.org/wiki/Island_Line,_Isle_of_Wight) states that...
"
passenger numbers ... have fallen rapidly to an estimated 1.31 million in 2014–15. This is the lowest annual estimate since 1998–99, and suggests passenger numbers have fallen by 22% in the last four years"
So, looking at RTT for the Esplanade, there's 66 services a day during the week, 66 services a day on a Saturday and 46 services a day on a Sunday.
442 services per week, so 22,984 journeys per year.
That's
fifty seven passengers per journey. Enough to fill a single decker bus, with a handful standing, but certainly not enough for a single carriage 153.
Now, you can argue, as some "believers" do on here, that every single journey matters and it'd be an outrage if we closed a failed station like Breich because there are unquantifiable social benefits that always magically swing the number in favours of heavy rail continuing to serve tiny markets (even if it may look like a sledgehammer/ nut situation). Fair enough if you are in that camp - there's no point in arguing with people like that.
But do you really believe that fifty seven passengers per journey (and falling) is "perfectly respectable"? How low would it have to get until you decided that it's just not worth the hassle of an isolated non-standard bit of the SWR franchise?
Is there a threshold at which you'd finally admit it isn't worth the candle? When it gets under a million passengers a year? It's been loosing a hundred thousand passengers per year in recent times, so it's not unthinkable that it'll go under a million before long.
(and I know there's the "but if we only invested billions in upgrading it then more people would use it" argument - I've heard the Ray Liotta quote more than a couple of times - but your actual point was about the current falling passenger numbers still being "perfectly respectable" and "enough" to warrant the existing service)