Er, there are fewer retail units (including the Suburban Ticket Office) opening onto the concourse now than there ever were, including in 1969. I remember it.
I'm not sure...
I remember that when you came through the barrier line (and up then ramp to concourse level from the suburban platform) you had a clear sight through windows/doors in the far side to the Euston Road. Although the new food outlets are on the upper level on the Euston Road side of the station, below them is a solid block which didn't exist, and occupies former concourse space.
Similarly, the 'East Colonnade' (the route from Evershoilt Street) was wider - the retail units have been extended onto it. the original building line was in line with the office entrance doors on the right (as you enter the station) that remains unchanged at the top of the steps.
When you enter the concourse from that East Colonnade you have to turn sharp right - I don't remember that constriction in the late '60s early '70s.
The upper level (where the Avanti 1st class lounge etc is now) was open to passengers and had more open space and seats, in my recollection.
Retail units on the station concourse itself was limited to the 'Four Leaf Clover'. On the East Colonnade there was Barclays (??) Bank, Wallace Heaton and Menzies on the corner., on the West one just the 'Bufferstop' bar. Exiting to the west you could also leave by passing between the Left Luggage Office and platforms 14-17.
Passengers in the 1960s/70s tended to arrive perhaps ten minutes before their trains' departure. At some of the present eating outlets - for example 'Nando's' - I would think you might have to arrive about an hour before train departure time have time to order and to eat. Having people waiting around obviously increases congestion, but part of the reason for this is that, now, you have to catch a specific train otherwise your ticket isn't valid.