I assume they don't like people on the platform when a train comes in (which makes sense for IC trains, but 'local' ones?) and they were likely also concerned at how to get the arriving passengers out. So I assume someone thinks you keep everyone on the concourse and then there's room for people to safely alight and leave, then you get people down to board.
If people are already being invited to board, or not staying on the concourse and crowding the gates then there is certainly a problem and I would say that the passengers who are crowding the corridors/gates are partly to blame, but are only doing it because they see 'boarding' and fear missing the train. This is why I guess they hide trains, but then people can still see them on OTT and other apps (or knowing the layout of the station and which platforms can be used and which can't).
I'm not sure what the solution is, but it would seem that you'd perhaps make the exception for the Overground trains and allow people to enter, to clear the queues. However, I am sure that it won't take long until some passengers see an opportunity to get in and then wander around for other trains.
Euston is clearly a mess. It relies on so many things to get right, and once things are out of control I can't see how you recover unless you close the station and force everyone outside (but risking a mini riot). Euston needs to take into account passenger behaviour, as they can't just hope people will hang back to make room. It's a dog eat dog world, so everyone is looking to get an advantage.
At the very least, perhaps announcements should be made to explain the situation and that nobody will miss their train - please keep clear to allow people off the train etc etc. Those without earbuds might respond positively.