You may be right but I can't recall seeing that suggested before, it's always been my understanding that the Cowes line was only ever considered in addition to the Shanklin/Ventnor line due to it's relatively low use.
As someone who moved to the Island in 1964 (St Helen's) and Cowes a year later I've seen the changes.
In them ol' days BR tended to route people via Portsmouth whether they were going to Cowes or not. However it was a bit illogical to travel from Cowes to Ryde then get to Pompey and on to Waterloo, when one could go to Southampton and then to Waterloo that way. Of course the Southampton, Isle of Wight and South of England Royal Mail Steam Packet Company Ltd (aka Red Funnel) didn't feature too highly on the rail timetable in those days.
With Newport I suppose on balance it probably would just have been easier to go via Ryde than Cowes. We had the COR units then on the Portsmouth line and Southampton wasn't electrified until a tad later so Ryde would have been quicker. Thus the flows on the Ryde - Newport - Cowes line would have been different to the Ventnor line because unlike the latter it wasn't predominantly one way up (to London via Ryde) and the other way back down.
Again look at the siting of the stations. With the exception of Cowes they were all (and still are) lousily sited in relation to the towns they serve(d)
Ryde Esplanade - ok but mainly a pier access point at the foot of a big hill
Ryde St Johns - middle of urban nowhere in the bottom of a dip with an uphill hike to the High Street
Sandown & Shanklin both at the back of their respective towns
Ventnor - Half way up St Boniface Down
Newport - At the back of the town but not as bad as Sandown or Shanklin but a 5-7 min walk to the middle
Cowes - best sited, a minute walk from the High Street.
The passenger role of the railways was largely getting you on or off the Island not really serving local needs to/from the retail areas. This clearly puts it at a major disadvantage. Almost like say Southampton Airport Parkway. Great for commuting/going to London or the airport, not so good for Eastliegh (which of course has its own station). This reduces the attractiveness of the network.
As I posted earlier if you had the network now that you had in 1950 you'd probably centre it on Newport, rather than Ryde. I think the Islandline is a good example of Geographical inertia in a transport sense.
We have the same problem up here with my local line to Stranraer. Great access point for Ireland but now the ferries are six miles away traffic levels aren't great, hindered by expensive fares and an ineffective tourism marketing organisation.