Nobody's suggesting closing the station, and the Haddenham and Kidlington examples you quote are cases where the situation changed thanks to significant residential development. Cherwell council seem keen on building houses by the thousand, so that's maybe the answer.
Incidentally, how do you see things developing once East West rail is up and running? A potential for more trains for Islip (since there won't be any conflicts at the London end to worry about) or increased demand for longer distance services creating another threat?
(As a side note, interesting that Charlbury came up in the discussion, as the first time I used that station it was to walk to... Islip!)
At Kidlington the development of houses has made no difference at all, despite the county council having a long-standing vague plan to reopen the old station, rather like its vague plan to reopen Wantage Road. Even now Chiltern's interest is not really in serving Kidlington, as can be seen in its choice of name for the parkway station at Water Eaton, which leaves out Kidlington and, rather like the existing central station, is a hard place to get to from much of the city of Oxford at busy times of the day.
As I said previously there is no provision in the draft Cherwell local plan for housing development in Islip, apart from the odd bit of infilling. The focus up to 2031 is on Bicester's dubiously-named eco-town, Banbury and the site of the former US Air Force base at Upper Heyford.
And as I also said previously, I expect East West Rail will be seized upon by Chiltern as an opportunity to get out of any obligation to serve Islip, unless some sort of steady peak traffic to London and back emerges from next year to justify the odd Islip call by its trains, same as I confidently expect that if a revived GW London-Stratford service ever comes to pass, Chiltern would hope to get out of any expectation to serve Stratford as well.
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Would seem odd to run Islip down for the future, especially when 194m platforms are being put in and the signals are spaced to allow them to be extended further than that.
Ascott-under-Wychwood got a shiny new platform and an extension to the other one in 2011 during the Cotswold Line redoubling. And until now has got precisely no more regular trains than called there previously, unless you count the odd pre-Christmas special Saturday stop for shoppers. There might finally be some extra regular services from next year, but nothing is confirmed as yet. Even then, will they be at times likely to make use of the train a more viable proposition for more people in the village, or will they stick with the drive to Charlbury?
At Islip the fear is that the train will no longer be a viable option if Chiltern proceed as proposed - and having 194m platforms won't matter if the train service offered is not fit for purpose.