If the road goes through Winslow its effects will be felt as far away as Wendover
I think the lack of an Aylesbury bypass and the distance beyond Aylesbury to actually get to the new road will stop that being the case. I doubt much Aylesbury traffic would use it (OK, those north or west edges of the town might, especially given the lack of bypass to get quickly across), given corridor B, and for traffic that would additionally have to drive through Aylesbury to get there...
There's zero reason to use the E-W expressway vs going existing routes to get to the M40 or M1 from south/east of Aylesbury - it will be slower regardless of traffic. The only bit that might be of any use is a south of Oxford route.
All the A roads will see a build up of traffic that will quickly overwhelm the new road.
Because there's massive demand for people to make journeys via that road? Seems like it's worth building then...
Of course, build the railway and make it decent (electrification from day 1 was a forlorn hope that came from basically one guy at the DfT whose 'Electric Spine' concept has long since been scrapped), but it shouldn't be either/or.
I'd argue that, because they put the road along corridor B rather than serve Aylesbury traffic, they view the Bicester-Bletchley corridor along the railway as important. That only makes sense if the railway itself is seen as important, else they would have gone via the large town.