@david_megginson @mekkaokereke Sure, but what happens when this person quits because they're working 14 hour shifts seven days a week because nobody can get anything done without running it by them first? I have seen these people get a coworker coming to their house and pounding on their door to get them to login to help with something on their day off. That shit gets bad very fast.
So perhaps your org can afford to say that this task is getting delayed a week because that essential person is out...but otherwise, if deadlines aren't gonna move for them alone, if whoever is there at the moment is going to be pushed to get that info however they can, then you need a different solution then just letting it all sit on one guy.