Given the extent to which 3+2 seating was used on London commuter services which have always been way busier than anything in Brum or Manchester l must disagree with your comments in that respect.
If one takes your argument to it's logical conclusion then WMR should have bought 710s with longitudinal seating.
Siri, show me someone who's never tried to get a 3 car 323 out of New Street at 6pm on a weekday...
Literally. I for many years (until 7 months ago) commuted almost daily between Victoria and Clapham Junction in rush hour. A Southern unit with 3+2 seating would often turn up. Not once, in either direction, was that train ever at 100% capacity inside and leaving passengers standing on the platform unable to board, and I was boarding at the final & busiest section at that too. Admittedly I've only used the CrossCity line thrice since moving back to Bromsgrove due to the post-covid reduced frequencies and unreliability due to strike action I now always drive as default, but on every occasion there have been poor passengers who try and board at Five Ways around 17:30/18:00 who cannot, as the train is 100% capacity people falling out as the doors open, and are thus left on the platform facing a potential 22-minute gap until the next service.
3+2 seating is perfectly sufficient for the Southern units which are not overcrowded and do not need more standing room.
3+2 seating is not sufficient for the WMR CrossCity line units which are very much overcrowded and need more standing room.
As for the '710 longitudinal seating', again, no, that is not the logical conclusion by any means, otherwise WMR would have put it in.
Suitable seating layout and seats to standing room ratio is dictated by the balance of passenger levels, crowding, and average journey length.
The London Overground has very high passenger levels, high crowding, and short passenger journeys. That matrix makes it effectively a tube service meaning longitudinal seating is the most appropriate. If one looks at a line like the Elizabeth Line or the Metropolitan Line, we see the matrix change slightly with a greater proportion of longer journey lengths being made, but still plenty of short journeys being made in the central core, hence the switch to having a mix of normal and longitudinal seating.
2+2 seating with wide door spaces, as is the norm on most suburban London metro lines like Thameslink, and the inner-suburban SWR and SouthEastern routes, is the most appropriate for the CrossCity line; as the experts who designed the specification for these trains know very well.
