A train with a decent amount of seats in it cannot run through central London except on a limited stop/limited access basis. This is why the A-stock has not been replaced by something similar. It does not cope with the congested sections of the route from the centre out to Finchley Road, so the new trains have been replaced by trains with more free circulation space, especially around the doorways. The same trend can be seen if you compare the 1938 stock with the latest tube trains or the rebuilding of the 1973 Piccadilly Line units.Long Distance does not equate to Long Journey times neccesarily.
The Class 395 manages 57km (~35 miles) stop to stop in 19 minutes.
And Class 378s are unneccesarily extreme for most "metro" services.
I am talking about average journey times less than an hour, not the ten minutes of the Class 378s/S-Stock.
The down side of the S-stock is that people from further out are complaining about not being able to get a seat. Which is an echo of the same complaints that were raised when the A-stock was introduced in the early 1960s.
One size will not fit all.