OK, we know that logically the defending team shouldn't and wouldn't want to collapse a "scrum" or "maul" once it goes in-goal, but I think CTPEs point was what happens if they do it anyway? You can't give a PK for an offence which doesn't exist, because the scrum and maul finished as soon as the ball was over the goal line.
My logic is if collapsing the maul is dangerous play on the FoP (and don't forget that the IRB toyed with the idea of allowing a maul to be collapsed just 3 or 4 years ago) then the exact same action in-goal is also dangerous play. Different offence - but the same sanction.