I Blame the Sailors
The plan is to have two Umpire boats per race, with two Umpires in each boat. This usually works well, at the average event, but when the racing gets hot it is not unusual for the race to split into three separate pairs and with the best will in the world there is no way that two Umpire boats can cover this situation.
At a 'normal' event we have the flexibility to call for help and often there is another Umpire available, as other matches resolve themselves to simpler situations. This will not be the case at a Worlds where protocol is set and you do not deviate from the plan. 'Normal' events have tended to go to three umpire boats for the final knock out stages anyway.
The question is will it matter - will there be many closely fought races where three pairs get widely separated? Will the sailors feel cheated or just accept it? Who knows?
Given that cost is an issue would it be better to have only one Umpire in each Umpire boat (and then put three boats on each race)? Are they unbiased, do two eyes help you see more? Do both umpires have to see the incident - if you are trying to cover with only two umpire boats it is inevitable that they have to split their attention. So you are going to have an individual Umpire making the call anyway. We had some races where there was only one Umpire in the boat due to the need for a comfort break - I would guess that the Worlds will have a 'relief' Umpire available.
We do our best but the sailors do tend to push things.