The shuttle would be good, as it would allow the halts and Hanborough to be served without stopping as many Hereford / Worcester trains, reducing overcrowding for BOTH sets of passengers, and pleasing both.
But as I've said before in various threads, the Hanborough and Charlbury stops in the longer-distance trains will be staying, because of the number of people travelling in the peaks from these places to Reading and London and back, traffic that has built up due to through services.
While Hanborough station may look like a glorified halt, it is a railhead for a large area north and west of Oxford - and far more accessible than Oxford station, which is why people use it. Having to change trains 10 minutes after they've set off or just 10 minutes from home on the way back isn't likely to be very pleasing for commuters from Hanborough and
FGW▸ will want to keep them happy and keep them using its trains.
As the initial post about a shuttle service said, the apparent key aim is to generate more local traffic and to try to persuade people travelling into Oxford not to use their cars. A half-hourly service would spread out the peak loadings between Oxford, Hanborough and Charlbury, in particular, which will help ease pressure on the long-distance trains, but I don't imagine it is intended as a substitute for existing stops by the London trains.
How about cheaper fares North of Oxford only?
As I said, because the Cotswold Line trains have to serve two markets - you can't fill up a train with passengers going beyond Oxford on cheap tickets when it's also providing part of the half-hourly service between London and Oxford, so must have space available for passengers making that journey as well. This was the reason for Turbos and then Adelantes getting the push from the route - they don't have the capacity to cope with demand from both these flows much of the day.
The result is, of course, that
HSTs▸ run with empty seats past Oxford, but that can't be helped, unless you're going to try to drive up journeys ending and starting at Oxford, which a shuttle may be intended to stimulate.
For cheaper off-peak local journeys between Oxford and Worcester there is of course the Cotswold Line railcard. It is to be hoped that if the shuttle does materialise, it will be heavily promoted in conjunction with this railcard.