That is not at all what I was referring to. You know how Theo Bos conducted himself in the sprints, right?
And an obvious problem is that the battle for position would just become even more fierce. You just can't control road racing like that, putting lanes in the final 300 metres...
I do agree with you that punishments based on consequences rather than actions are undesirable.
So my issue was that in general, I try to debate the proposal, not the proposer. But also What Libertine Said™. Bos knows as well as anyone what would actually dissuade riders from doing dodgy things.
It would be rather apt, though. I mean, David Millar got to sit on the UCI's anti-doping panel, because of his experience of doping making him more valuable to it than clean riders (thus gaining yet another unmerited advantage), so the same premise suggests Theo Bos should sit on the UCI's anti-sprint-crashing panel, because of his experience of causing sprint crashes.
I also think that this rule is not going to prevent crashes, just move them a few hundred metres up the road. Where in smaller races there might not be barriers. And how big does a group need to be for it to apply, are we going to see an end to two-up sprint tactics entirely?
As to your question, I think you might be misunderstanding the proposal. You can switch lanes to pass someone. "If you overtake riders from behind, you are allowed to leave the box, otherwise you cannot pass in it." There should be no issues with this vis-a-vis a two up sprint, you it just makes it a bit harder to try to cut someone off, which I think is warranted. The Van Aert and Vanderpoel sprint would be a great example of this. The tactics would be exactly the same, and the sprint would have been the same. This way it just would have made it a bit more obvious had Van der Poel deviated, even though the rules would have been basically the same. If anything you would get more sprints like that, where the tactics play out over a slightly longer stretch and riders would be less inclined to do nervy swerves at the end to hold their leads.
But as to bunch sprints, which is the main point, I think both of you are off on this. The leadout guys are the ones positioning the sprinters up until 300m. I really doubt you"ll see five trains going on simultaneously, you would likely get a couple of trains going and then the fastest guys going at it pretty much as is. Yeah, there would be more of a jostle for position at 300m, but I think that would not be that much different from what happens nowadays anyway -see Sam Bennet at the Vuelta. This would not prevent that, unfortunately. Maybe the emerging trend of harsher sanctions will, or further rules on cutting off riders.
The two things this is trying to prevent, as I see it, is: sprinters who are leading or side by side with other riders, and leadout men who drop off (a point that Bos makes that I had not really thought of) from swerving in any way; and riders from going for gaps -particularly against the barriers- that do not exist. You would just have more consistent sanctioning of that, regardless of outcome. Otherwise, sprints should not change that much. They would just be safer.
Maybe there are other issues that come up. The devil almost always lies in the unintended consequences. But I wholeheatedly believe that something like this warrants a trial.