As
@AlastairFraser says, congestion on the A40 approaching Oxford (never mind Oxford itself) is such that "carrying on as they are" isn't really a viable option most times of the day.
Oxfordshire County Council is about to start work on Shores Green junction (
map) to add west-facing sliproads, with the express intention that people on the east side of Witney will be able to use the A40 to access the Ducklington roundabout and similar areas, rather than having to fight through the one river crossing in the centre of Witney.
For Eynsham I believe the current railway proposal is indeed to reroute to the north.
But even if the line were ever (re)opened, Witney/Carterton would work as the terminus either of the hourly non-Cotswold IET from Paddington to Oxford, or of the 387 electric stopping service. It wouldn't really make any sense to link it into EWR.