The preferable solution would be to replace the 317s & 321s with the 377/5s, with the 377/5s running the fast Cambridge services, and the 365s everything else. Can you, honestly, say the 700s are better than that combination?
I doubt anybody knows what the final plan is yet, but as you asked my thoughts are:
My commutes were to Kings Cross (4 years) Liverpool Street (6 years), Victoria (5 years) and Blackfriars (South of river) 5 years.
If I was still living in Biggleswade my commutes would certainly be better than they were when I started in 1983, no wondering if the 03.15 staff DMU would turn up. . .
From a non enthusiast perspective . . . .
The 317s and 321s need to go, end of. I concur that 377/5s would be better than 365s. I regularly use 377s now and would choose them any day over a 365. The 365 seat pitch is awful and the recent suggestions here that people use the tables on them for work is a bit ambitious. Yes they are useful if you are having a cuppa but not much else,. My commute varied from reading a magazine / book through to listening to a bit of music whilst slurping a tea on the way home and on much quieter trains maybe enjoying a snack.
If I worked at Liverpool Street I suspect that Farringdon would be my change of choice, but back in the day changes at Finsbury Park / Moorgate on the Southbound were often taken when there was a 313 running in, Kings Cross the much more common trip.
Victoria? Bit of a no brainer, whatever train, change at Finsbury Park Southbound and Kings Cross Northbound. If you avoid the suggested routes the walk from the Victoria line is OK to the Kings Cross platforms and a bit of a pain to St Pancras low level.
Blackfriars. . . God if only that had been in place back then. My preferred commute was train to / from Kings Cross and then the 45/63 bus, occasionally a Thameslink with a stroll over the bridge when i was flush for time. Going home was a nightmare. All my shifts ended on the hour so I never made the fast trains home at xx.21 and was often too tight for comfort for the xx.36 and always planned to be on the "next" xx.21. If I could walk onto a through train home, or even change at Stevenage / Hitchin / from Blackfriars to wait the next train surely that is a no brainer?
Now back to the question. Let's say I am back to my regular commute? For ease of boarding and decent on board loos that are not cramped (I am old enough to need them too many times now) the 700s win hands down over anything that's out there (the small loos have sliding doors, not doors that mean you have to squeeze awkwardly). The seat pitch is certainly better than 365s and less cramped, so I guess a 700 wins for me.
The whole idyll of getting to Kings Cross and getting on a train in the station disappeared years ago when Major stations took over the control point and boarding of trains turned into a lottery, made worse by the recent addition of the gate lines and the last minute scrambles that are prevalent now.
At least if you are at Farringdon or St Pancras you know where the train will be and with knowledge of how late a train is at the tip of your fingertips, any aversion to platform waiting is easily overcome with tthe numerous diversions of retail outlets.
What is the genuine hardship that you fear?