I actually want them open as a visual clue that there's no room. Far easier a truth to assess than seeing one closed and not knowing if someone was just being a dick.

Yes. Passengers are clearly untrustworthy. I almost never see the bins packed like that and usually people close when there is still room

The thing is that many “carryon” bags will not fit like that but it shouldnt matter. Even laying down there is usually room for three bags to match the number of seats.

Ive frequently see passengers close the bin when there are bags laying down the long way, wasting excess space whne there is clearly more room

It depends, sometimes better efficiency can be achieved if bags are shifted around and letting the attendants see them in a glance can help that process faster