Birmingham trains on Chiltern do often leave Marylebone full and standing by my observation. Late evening can be particularly bad e.g. 2208 2 coaches 2233 3 coaches often go out with people stood down the aisles. However as mentioned by others they're currently providing the faster services for surburban stations at off peak times and I suspect by Haddenham and Thame Parkway / Bicester at worst they've emptied out. But this doesn't offer a great option for someone travelling to Birmingham who potentially has to stand for the first hour or so.
However I suspect passengers travelling the full distance at that time of night are few in number and it's not ideal providing extra carriages to carry fresh air all that way. Ideally the Birminghams would be fast to Bicester North or Banbury and extra semi fast services added to follow to cover the intermediate stops. This was what happened in the original "Evergreen 3" timetable but was gradually eroded to best utilise very limited carriage capacity and accelerated post-COVID to reduce the numbers of services operating
However I suspect passengers travelling the full distance at that time of night are few in number and it's not ideal providing extra carriages to carry fresh air all that way. Ideally the Birminghams would be fast to Bicester North or Banbury and extra semi fast services added to follow to cover the intermediate stops. This was what happened in the original "Evergreen 3" timetable but was gradually eroded to best utilise very limited carriage capacity and accelerated post-COVID to reduce the numbers of services operating