It is (and is a journey I've made myself!), though would risk importing delays and those people could change at Risborough and thus avoid that cross-pollination between the main Chiltern line and the WCML. (The Aylesbury line isn't much of a concern because it isn't used to anything like capacity past Amersham).
Based on the shadow timetable, passengers will be able to get from High Wycombe to Claydon West Junction in about 50 minutes, with a 15 minute change at Bicester Village, and then on to Bletchley and Milton Keynes.
Running times from High Wycombe via Aylesbury to Claydon West Junction would likely be around 45 minutes on a direct service, or 50 minutes with a quick 5-minute connection at Princes Risborough. So unless you're running through from High Wycombe, it's unlikely to be any quicker changing at Princes Risborough for a train via Aylesbury than it is using the main services via Bicester Village.
On the topic of Buckingham, wouldn't it make it a lot easier to reopen the branch to Buckingham now EWR is being built. Buckingham would really benefit from a rail connection again as it is quite a large town lacking a rail connection so there will be a decent commuter market, plus students going to and from the uni
The only place of note on East–West Rail that Buckingham doesn't already have a decent bus to is Bletchley. Winslow is a good railhead, although for passengers going on the X6 bus to Winslow then unless they are going to Bletchley then it's unlikely to be quicker than just using the direct X5 bus to their destination.
Thinking about it quite a few large towns in Buckinghamshire don't have a rail connection. Brackley , Towcester , Buckingham and Daventry.
Silverstone could also benefit from a rail connection due to its race track but i dont think silverstone has a rail connection that would be possible to reinstate.
(Pedant mode: Buckingham is the only one of those that's in Buckinghamshire, the others are all in Northamptonshire)
With Buckingham and Brackley each having a population of about 15,000, they certainly aren't insignificant, but I'm not convinced that a new rail line would be the most effective investment, largely because there isn't one obvious destination. The old route from Verney Junction via Buckingham and Brackley to Banbury is largely but not completely unobstructed, so some significant civils and/or compulsory purchase would still be needed – but I suspect people's destinations are quite widely dispersed and a lot of them won't be well served by a line going to Milton Keynes in one direction (that is no quicker than the bus) and Banbury in the other. I'm not saying the line doesn't have merit, but it would be hard to prioritise it over other lines that would be likely to carry more traffic and have a more beneficial impact on economic regeneration (which is not something desperately needed in rural Northamptonshire!).
Daventry definitely does deserve a rail connection, but the question is logistics – which one? You would struggle to get a station anywhere useful, right on the edge of town is realistically the only option. The easiest previous route to reinstate would be direct onto the West Coast Mainline at Weedon Bec, with trains down to Milton Keynes (and potentially beyond), but that misses the nearest big town which is Northampton. The surrounding area is quite hilly, which would make any new line tricky – my preferred route would be from the north-east corner of the town out along the same corridor as the canal, joining the Northampton loop just south of Long Buckby, but I can't see it happening.
Towcester is smaller, and harder to serve with any kind of useful line – and Silverstone wouldn't generate enough traffic consistently through the year to merit a dedicated line to it, and I can't see a sensible through route that would pass by.
