Fully appreciated re. why barriers are beneficial, and if crowding on the platforms is leading to PTI issues then ticket checks absolutely need to go out of the window for safety reasons, I agree.
That should happen irrespective of whatever onboard ticket checks have taken place, though, and the extent of those checks won’t be known (or cared about) by the gateline and control room staff making the decisions.
EDIT: looking again at the photo in the OP, that level of crowding to exit the platform does look absolutely ridiculous. I’ve never seen anything like that at any of the various London terminals I have signed, thankfully. Euston really does seem to have “issues” all of its own…