Absolutely this. Anyone who's alighted from a well laden train arriving at King's Cross will know how treacly the flow of people is down the platform and through the barriers. Force that flow (from two directions) up to the bridge and through the narrow entrance onto the mezzanine will be a recipe for chaos.That might be ideal but it's not practical. How do you link a bridge at the north end back to the concourse?
Also the platforms are quite narrow. The existing lift and stairs reduce them considerably but are themselves far too narrow to cope with a trainload of alighting passengers safely. Even putting them at the platform ends wouldn't work, some are very narrow and people would still have to get past the stairs to get to the lift (or vice versa) or to access the bit of the platform beyond as the ends don't line up.
The station is infinitely better as a passenger than pre-rebuild, and where there are issues with flow, they are issues to do with having multiple trainloads trying to cross each other.