The 275 does seem to have long been among the weakest, if not
the weakest inter-urban service leaving both Oxford and High Wycombe, so it doesn't come as too much of a surprise. Presumably there is more to it than just Carousel being based in High Wycombe for those positioning journeys, and they may actually be the ones carrying most passengers? I think we have to remember that what looks like a good line on the map may actually be carrying fresh air and the driver over parts of the day, and with modern ticket machines, that's easy to tell.
How many inter-urban services now only run Monday to Friday? Whilst we're used to seeing that on village-town services, it seems rather more unusual on town-town services such as this, which is perhaps another giveaway.
Bledlow Ridge, population ca. 900, is in Buckinghamshire, and as the operator states to have worked with Oxfordshire on a new timetable, and the registration states it is part subsidised by Oxfordshire, that seems to suggest Bledlow Ridge is a Buckinghamshire problem. In any case, it won't be cut off completely, as it still has the Princes Risborough Community Bus (and presumably, connections in Princes Ris. to High Wycombe if wanted).
I had it at the back of my mind that there had previously been a truncation of service, and a bit of delving seems to suggest in 2003, it operated with a peak journey Mon-Sat, but off-peak journeys only ran Mon/Wed/Fri, and even then Mondays and Wednesdays it terminated at Stokenchurch (when the whole service ran that way; rerouted via Bledlow Ridge in 2012). I think it was interworked with the 111/122/124 shoppers services at the time, to Watlington, etc.?
(Note: Direct download of Word doc, but as source:
https://www.oxford-chiltern-bus-page.co.uk/275 tt 070703.doc).
As others have suggested, I suspect the suggestion of travelling using the Airline service is just to avoid mentioning the competition (although the 40 also looks to be at least part supported by Oxfordshire CC, so not really competition per se), not that they are actually expecting anyone to use it. After all, if there were enough passengers to make it viable, they wouldn't be withdrawing it!
Oddly, one wonders why it doesn't get introduced on Saturdays with some whacking great marketing advertising it as a day out from High Wycombe to the City of Dreaming Spires... I suspect the interest really is very low!