That is oh so easy to say. This is the moment that Sagan decides to start sprinting for the gap:
Seems to be a gap. A tiny gap, but a bigger gap than Ackermann had to be sure. This is the moment Wout drifts slightly left (not that it was intentional, but there he is):
At this point what do you do? His momentum is carrying him into van Aert. He can slam the brakes and risk being crashed into, and will likely bump into Wout anyways without any chance to position himself as he is reaching for the brakes. He can crash into the barrier. Or he can bump into Van Aert. He has milliseconds to decide.
Again, the risky move is taking the line at all. The rest is just punishing riders for the consequences, not their decisions.