Operators set standards for the network. Passengers expect those standards to be maintained. The Standard class seats for the 700s are hard, and do not have armrests or tables, yet they are in a 2+2 layout. By getting rid of seats less people can sit (obviously) and the people that do get to sit will have to sit in a lot less comfort as currently all BML stock have armrests and/or tables bar the 319/4s, and at least the seats on those trains were comfy.
The First Class is also appalling. I know that the Brighton Mainline isn't the best for First Class so we could let this one slide, but the 319's had an excellent 2+1 intercity style FC with large tables and fairly comfortable seats. The 442's also have a very good FC section, and the 377/4s and /5s have FC in compartments with slightly bigger seats, more legroom and power points. I'm not sure if the 700s have plug sockets, but what I can say is that FC passengers will be greeted with ironing boards for seats and a layout that should be used in standard, a far cry from the previous 319's.
In other words, most of the interior needs upgrading