Certainly where I am, as we approach London, the guards will remind passengers that barriers are operating and that they need to have their tickets with them to exit the station, irrespective of the fact they’ve already been checked. That’s not to say they shouldn’t be checked en route, of course, but that will make no difference to how quickly you can get through the barriers.
I don't think that an exactly comparable situation arises that people from one train are just going to one exit that only applies to those specific platforms; furthermore, I'd like to think that the barriers would be opened in the event that it caused a crowding issue. Indeed
I have seen this happen at the London terminus I believe you may be referring to, myself, where a pragmatic approach
was taken, rather than an attempt to check every ticket.
Incidentally, I did observe a safety incident at that same terminus, so the problem can happen anywhere, however the reaction of the TOC staff (not the agency staff) was brilliant and impressive and immediate steps were taken to intervene and rectify the matter. I would have absolutely zero confidence that such an approach would happen at Euston; indeed I am aware of people who expressed genuine concerns at Euston, in my opinion in an entirely appropriate manner, having had the police called on them for raising the concerns (and no I cannot say any more than that, at this time)
When the seriousness of attempts to silence people at Euston are eventually exposed, people are going to be really shocked at the scale of the inappropriate behaviour by Network Rail staff at Euston, and also at a higher level within the company.
If gateline crowding is genuinely reaching dangerous (as opposed to mildly inconvenient) levels the barriers should simply be opened, of course, and no ticket checks would take place.
While it is difficult to conclusively say this specific situation was genuinely dangerous, there are so many concerns over Euston (and I am privy to more information than most, which hopefully will be disclosed once a legal matter has concluded) that it's not unreasonable to conclude that there is a good chance that such concerns are well-founded.
In any case, the train was already delayed and passengers incurred a further delay of a reported ~10 minutes (assuming the claims are true, which we have no reason to deny) to passengers affected, which really is diabolical customer service, even if there isn't a safety matter.
Indeed - incentives can be provided, or there can be a minimum revenue figure “target” imposed, as our IC guards are subject to. This can lead to some not bothering once they’ve hit their target, though. They don’t get any commission from sales, so there’s no benefit (to them) of checking any more!
I'm not quite sure if we can conclude that a lack of commission means that a correct and pragmatic approach will always be taken, but that's a whole new topic.