Class 377 and class 700 are both fit for purpose. They are very different because their purposes are very different.
Most London commuter services are in and out of London termini. At the termini everyone is getting off inbound services and getting on outbound services. When those trains stop at other London stations, for example Stratford or Tottenham Hale, it is still the case that most people are getting off inbound trains and getting onto outbound trains. Here the distinction between short and long journeys determines the layout: 3+2 provides more seats on longer distance journeys, where most people want to sit down. When the class 377s were new those seats were needed on commuter trains to/from the Sussex coast. The clearest new example of this is the 3+2 seating in the GA class 720. Space for circulation of passengers is not important because, when people get on or off crowded trains, nearly everyone is moving in the same direction, and at the termini time is not a constraint.
What makes Thameslink different is the core, a sequence of 5 stations where there are lots of people getting off and lots of people getting on the same trains. In order to maximise capacity through the core it is necessary to minimise dwell times in the core. With the 24tph objective that requires dwell times of no more than 1 minute. Most of the distinctive features of the class 700 are designed in to minimise dwell times in the core. In particular the space in class 700s is dynamic, for people to move around in, not static, for them to stand up in.
At busy times there is a large contingent of frequent travellers who are "nudged" into behaviours that help to reduce dwell times. One of those is for all of the trains to look the same, which is why a class 700/0 on Metro workings has the same layout as a class 700/1 on long distance workings. The precursor of the class 700 is the SWR class 455 refurbishment, which incorporated many of the design features of the class 700. That's because the Clapham Junction Windsor Line platforms, Wimbledon and Richmond come closest to replicating the Thameslink core challenge of minimising dwell times when lots of people are getting off and lots of people are getting on the same train.
Most London commuter services are in and out of London termini. At the termini everyone is getting off inbound services and getting on outbound services. When those trains stop at other London stations, for example Stratford or Tottenham Hale, it is still the case that most people are getting off inbound trains and getting onto outbound trains. Here the distinction between short and long journeys determines the layout: 3+2 provides more seats on longer distance journeys, where most people want to sit down. When the class 377s were new those seats were needed on commuter trains to/from the Sussex coast. The clearest new example of this is the 3+2 seating in the GA class 720. Space for circulation of passengers is not important because, when people get on or off crowded trains, nearly everyone is moving in the same direction, and at the termini time is not a constraint.
What makes Thameslink different is the core, a sequence of 5 stations where there are lots of people getting off and lots of people getting on the same trains. In order to maximise capacity through the core it is necessary to minimise dwell times in the core. With the 24tph objective that requires dwell times of no more than 1 minute. Most of the distinctive features of the class 700 are designed in to minimise dwell times in the core. In particular the space in class 700s is dynamic, for people to move around in, not static, for them to stand up in.
At busy times there is a large contingent of frequent travellers who are "nudged" into behaviours that help to reduce dwell times. One of those is for all of the trains to look the same, which is why a class 700/0 on Metro workings has the same layout as a class 700/1 on long distance workings. The precursor of the class 700 is the SWR class 455 refurbishment, which incorporated many of the design features of the class 700. That's because the Clapham Junction Windsor Line platforms, Wimbledon and Richmond come closest to replicating the Thameslink core challenge of minimising dwell times when lots of people are getting off and lots of people are getting on the same train.
