Oxford is bigger and more famous and all that - I'm not doubting that. I said: "if Oxford deserves 3 or 4tph to Birmingham (which I don't doubt) then Wycombe deserves, at the very least, to keep its 1tph!"
Even with your understating the importance of Wycombe, it surely is worth a third of Oxford? Especially as we're also adding in Leamington-London traffic, etc.
How about Watford as a comparator? Watford doesn't have a uni (unlikely Wycombe), is a similar size to Wycombe (about 7% more population), is inside the M25 (and the London Travel to Work Area, whereas Wycombe isn't as it's its own place), yet it has an ICWC service (plus the London Midland services) to Birmingham. How absurd would it be if, when London-Birmingham long-distance traffic is removed from the WCML and a more regional operation can take place, Watford lost its direct service to Birmingham and, you had to change to get north of Northampton. That would be crazy! But that's effectively what you are suggesting for Wycombe by turning back the clock to the mid-90s and terminating everything from Marylebone at Banbury.
They probably would if Meerkat's silly proposal to force Leamington-London traffic to change at Banbury (or double back via central Birmingham) happens.
Or, more likely, they'd just drive south.